There are several issues I try to balance against each other when determining mic placement in front of an orchestra. You solidify a ratio of the front of the orchestra to the back of the orchestra in your choice. You are also making tonal decisions since the strings radiate with a drum head like complexity so they tend to sound different at different heights. Of course you are also deciding on the direct sound to ambience ratio as well. Also, when there is an audience involved, the further into the hall, the more audience noise you will pick up. Balancing those major factors is the art form of it.
I am surprised that you notice micing further into the hall when there is an audience. I believe this is probably due to limited hang points or bad choices. It's hard to if not impossible for me to say I want to be this high and this far out in any given hall since the rooms all have different effects on the sound. Ok, maybe as a starting point, but it will likely change if I get a sound check. Sometimes surprisingly so.
It is sometimes the case, even in new halls, that I don't get a sound check, then I use my best judgment and hope for the best. Hopefully you get a sense of the room when you setup in a new room that gets you in the tweakable ballpark. Hopefully also, you can then get the mics where you need them to be.