Is there a way to find out which fret position would be optimal? Or is it a matter of personal choice? (When no additional info is given in the sheet music)
In a word, no!
You'll start to find your way round, and realise that certain positions are better than others. It also depends on the sound you want. Playing up the 'board on higher frets and lower strings will give you a different sound than playing exactly the same notes lower, on higher (thus thinner) strings. At any given place, you can encompass two octaves without moving your hand up or down, so that's a good premise to start with.
Once you start playing this way, you'll realise that some people who write out tab haven't much of a clue, really...
There is some logic behind how we are taught to read and relate what we read to the positions on the guitar.
Most good method systems in classical or modern guitar, e.g. Carcassi, Mel Bay, Levitt, just to name a few, focus lessons on specific patterns and positions. Those are drilled for years until they are in muscle memory, and the mind immediately correlates what you see on the sheet music with a movement. That being said there is still some ambiguity and freedom of choice.
In the standard guitar notation we have special decorations (in the form of numbers, numbers in circles, and roman numerals) to tell the guitarist, (1) what finger to use to fret the note, (2) what string to use, and (3) what position to be in. This is usually the result of another guitarist arranging the piece in a way they thing is best for "easy", clean, meaningful performance.
Based on all the training if a piece of music tells you what position to be in the options for fingering are somewhat reduced. If the piece can be played in a single position, i.e. only goes through one or two octaves. They many players will peruse the piece and, based on experience, get a position and stick to it. But in reality that is not the best approach. With experience you learn to make smart choices quickly but if you are sight reading a very complex piece with no guitar arrangement chances are no matter how good a reader you are you will need to take some time to strategize.
Some of this strategy comes from understanding the CAGED system and the scale patterns associated with each chord in the system. Levitt's approach follows this to a good degree. Mel Bay focuses a lot on open position scales for many books, and follows Carcassi's classical method. In fact there is a lot of overlap in exercises between the two.
At the end of the day there is no rule for this. Just like on the piano, you could use one finger and keep shifting around. But there are some logical and illogical approaches to making a decision. One of the most common patterns in classical guitar is the standard Phrygian mode which corresponds to the C form of the major chord related to the key. There is no shifting in this mode and one can pretty much stick to the 1 finger = 1 fret rule for reading single note lines. Levitt adheres to this in the beginning and then introduces the "Finger Stretch" (fs) for deviating out of the 4 fret neighborhood. Once learnt you'll find that you can read through material pretty fast.
Chords are a bit tricky. There is more than one fingering for each chord but their use is not random. A choice usually requires understanding how to connect them to the chords around them for optimal movement. This requires previewing the music to make a decision. Using a "standard" form for a chord every time you see it can lead to getting locked into a position you can't get out of in time and just losing your place.
Not only is there more than one way to finger notes in a fixed position, there are several placed to play the exact same notes doe to the repetition on the guitar. The only real guideline there might be that you usually play melodies on the upper strings to be bright (but that's not a hard and fast RULE).
Personally, I haven't learned classical guitar technique. But I understand it works with formal positions. When you learn the positions, the fingering and fret/string choices are supposed to become intuitive. I imagine that "intuitive" sense of fret/string fingering only holds up when the music is in the classical style, when the music is based on the patterns learned in classical training. Classical guitar notation does have markings to indicate strings. So clearly there can be passages where fret/string aren't obvious without a fret or string marking of some kind. It just doesn't necessarily need to be fully tabbed out.