There cannot be any one answer. Every tuning system includes intervals which are out of tune, so you will never achieve perfect concord: how close you get depends on a myriad of different factors - how experienced you are, how much time is available, how finely an instrument is made, how stable it is, what other instruments you are going to play with, the weather, whether you have a restless audience in front of you or not: in fact all the factors present in the situation will affect the result!

Let us think about a guitarist for a moment, with his fixed frets giving him equal-tempered semitones, and his E-A-d-g--e' tuning. If he does what is natural, and sets his open strings to give him a lovely, well-sounding E major chord, then his G-major won't be in tune: the G which is three equal-tempered semitones above the bottom E is not a pure 3rd (plus an octave) from the open b-natural, but is too flat - and the open g is even flatter!

Similarly , if you set your open b-natural so that it is in tune with the E, and the g in tune a 3rd below the b-natural, then the g# you get, an equal-tempered semitone above the open g, will be very sharp with the E and b-natural.

The simplest solution must be to accept that a guitar is designed so that its open strings are no better in tune than a piano, and - since ordinary mortals have great difficulty in tuning equal temperament - tune each string separately to a well-tuned piano. If a guitarist is used to listening, and knows that he is likely to play in some keys more than others, or that he is going to play with instruments which use some other tuning - a violin using perfect 5ths, for instance, or a harpsichord using an unequal temperament - he can adjust his open strings by small amounts here and there, and he can also be prepared to vary the pressure of his left-hand fingers at specially sensitive points in the music.

As a guitarist you can play through the whole piece and listen out for which notes jar most, and see if you can improve matters by a small adjustment: the 'best fit' may well be different for different pieces - remember that a lutenist even accepts the need to move his frets sometimes for different pieces.

The important things to recognise are

1. that if you have fixed, equal-tempered-semitone frets, having in-tune open strings can only produce out-of-tune stopped notes: and
2. that tuning each note separately to a keyboard instrument is not a sign that you can't tune perfect 3rds and 4ths, but rather, it shows that you know something not everyone does, that perfect 3rds and 4ths don't work on a modern guitar - and perhaps even why!

Most people seem to think that tuning for an ensemble session involves producing an a' which is, with a minute degree of accuracy, the same as his neighbour's: but of course that's not what it's all about at all. You need to be sure you are all agreed on what tuning system you are aiming for, that it will work with those particular instruments in those particular keys, and for those particular pieces you're going to play: you need to check key-notes, 3rds and 5ths, you need to check accidental sharps and flats, to ensure that you all have the same ideas about how large your intervals are to be: and perhaps most important, you all need to gain some realistic idea of what chances each of you has of reaching the goal you have set yourselves. You must all recognise that people are different, that two people can hear things in quite different ways, that they may set priorities in different aspects of a thing.

Let us say that, of course, everyone is agreed on what is the ideal to aim for: all notes coming at the right pitch and at the right time and lasting the right time and started and finished off in the right way and at the right volumes and the right sound and affecting the listener in the way the composer and perhaps even the performers intended and all the rest of it; even so, when there's a lot going on, it is just conceivable that not everyone will be able to give his whole concentration to every aspect the whole time, in the same proportions as everyone else: and it might just happen that the bass-player thinks that getting the subtleties of rhythm is the most important just then, the guitarist gives most of his attention to continuity and communication with the audience, the flautist is rather preoccupied with beauty of tone, the pianist is concerned about fingering and articulation, and only the violinist realises that actually it is the tuning which is the only thing that matters.

You have to recognise that people have different perceptions of reality, and you have to be ready to discuss all these things in a group, without making one another feel inadequate: of course, it's not easy to sweep aside deep-rooted inferiority complexes and all the rest but it only gets better by practising.
Nowadays you can read whole books on the subject of tuning, you can buy electronic devices which measure intervals for you, you can concentrate your attention on counting the number of dissonant beats you can hear in a second: but none of this reflects what was normal practice in earlier times, and it puts the modern intellectual musician in a very different state of mind from that which was normal before, too.

As far as tuning is concerned, renaissance and baroque musicians armed themselves with as much basic knowledge as could be squeezed into an odd page at the back of a general primer on music: they trained themselves to listen - thinking first in parts, and only secondly in chords - and to react sensitively to what they heard: and then they got on with the business of delighting the ear, expressing the passions, and affecting the listener.