QcCowboy

it depends on the register difference you are applying to the score.

if your first and second violins are playing Bb and G (below the staff) and the violas are playing middle C, then chances are you won't notice any significant difference in the sound had you actually placed the instruments in their "normal" order.

the higher up the violas play, however, they will be shortening their string length. this is an important effect as it tends to create an effect of "raised tension". the effet will not be as pronounced between violas and violins as it might be between cellos and violins. remember that the viola is basically only a 5th lower than the violin. Basically, the exact same note played on a viola and on a violin, will be a bit more "tense" sounding on the viola because it is playing in a higher string position. the same note between violin and cello will have a more marked difference in tension.

violas DO have a characteristic "rough" quality to their timbre, which is quite different from the violins. the same melody played on the G string of violins and on the G string of violas will have a different quality. The difference is generally light, and not as remarkable as say the difference between the same note played on a horn and on a trumpet, or a flute and an oboe.

remember that the strings strive to blend together as a cohesive unit.

I would say that placing the cellos above the violins would create a stronger difference in timbre. the cellos actually gain in quality with the tension rising, while violas tend (slightly) to lose their characteristic sound.