QUOTE(Miss Ross @ Aug 12 2007, 09:57 PM)

You're correct, very few scores have markings on them. However, with string music the composer will often specify instructions such as 'sul G' (can be found in, as Sarah mentioned, Massenet's 'Meditation') which is an indication to the player of which position to use. Obvously, it's completely up to you whether you choose to do this; as you said, the player should decide in relation to the feel of the music. Markings of this manner are less commonly found in orchestral or mixed ensemble pieces (from my experiences anyway), and are more common in solo string music.

Yes, they're much rarer (in my experience) in orchestral music (though I'm not at all sure they would necessarily appear in full scores anyway) though I have come across examples. In a key which is so string-unfriendly I would suggest they would be very helpful (depending on edition, I have come across them reasonably often in small ensemble pieces, and relatively a lot in chamber groups, trios, duets; most of all in solo music) plus if you intend the piece to be very expressive, you might want to be more specific as sul [a string], for example, can make quite a vast difference to the effect of the piece IMO. Similarly, string crossing during a slur vs playing in a higher position - you may WANT the effect of string crossing on a slur if you were to hear it, or you may specifically wish to avoid it. Same with bow markings, if you are aiming at specific effects then you may want to look into this just in case there are particular points where one or the other bow mark is more beneficial to the effect you wish to create. (Things like harmonics which have been mentioned by others in relation to the cello part can make an even greater difference as their sound is quite different)
Yes, experienced players are more than capable of working out good fingering etc, but you, as the composer, know what effect you want to create, whereas some random string player doesn't unless you have specifically told them. It depends how much leeway you want to leave. And yes, you may decide that you want to leave it entirely to the players, and obviously that is in the end totally your decision,
but IMO, it is a good idea to KNOW what decisions you're leaving to them. If there are position suggestions etc which leave the player more attention to work on playing the music beautifully, that can only be a good thing!

Such markings by no means have to be all over the music, but a helpful hint at a particularly nasty string crossing, or a bowing marking that helps along a dynamic marking, etc, can be immensely beneficial.
Also, if one wishes to write for string instruments, it is simply useful in the long term to know what a violin or whatever sounds like in 3rd (or 5th, or 4th, etc) as opposed to 1st position, con sord, playing harmonics, etc.
Speaking as a long-term chamber and ensemble player it's usually obvious which composers have an understanding of the instrument I am playing, (regardless of whether they necessarily play it themselves) and a part that is either string friendly or where the composer (or editor) has realised that something is not as friendly as it could be and so has laid down some guideposts to make it more accessible - not necessarily due to the part being in a string-friendly key or whatever to start with, but just written/edited/marked/whatever with an understanding of the instrument - has always created better results in the end - in MY (obviously amateur but wide ranging) ensemble experience.

This is all, clearly, in my experience and in my opinion - obviously you can ignore it totally. It just seems to me to be a good idea to check such things out to make one's music more idiomatic.