As a Piedmont pilot, I have flown on the Dash-8 a lot! In the case of the -100 (we don't operate -200s anymore, so the 37-seaters are all -100s), I personally prefer row 9. Rows 1 and 4 are off limits, as they are both considered exit rows (though 1A is not). Row 9 is in the back, but even on a full flight deplaning a 37 seat Dash is quick, so it's worth it to get the quietest ride.

On the -300, exit rows are 1 DF, 2CDF and 11 ACDF; given that 11 is not available for you, I personally like 13 for the same quiet reason that I prefer 9 on the -100.