Nontraditional seating would be at the bidding of the conductor, and done
in order to get the best balances, given the acoustics of the hall and the
scoring of the piece.  Lately, in Milwaukee, I have been seeing the violas
where the cellos used to be, to the right of the podium and closest to the
audience.  The cellos are now behind them, where the violas used to be.
I can't say I've heard the difference.  The lower strings always sounded
good to my ears, with a warm tone.  This did not use to be the case with
the violins.  The hall underwent an expensive makeover a year or two back,
though it is still hardly one of the great halls; you don't want to sit on
the side of the main floor.  Before the fix, the violins sounded thin; they
are much better now.

Percussion carries, but it can take up a lot of floor space.  Over the
years I've seen a lot of different placements.

Jim Tobin