It can happen, can't say how frequently, and it certainly helps if you're only looking for one ticket. I got second row for the 2012 Porgy & Bess revival once. The times I've gotten front center orchestra seats have all been less than an hour before curtain. Currently, Groundhog Day seems to release 5/6th row center orchestra seats at that hour mark.
Just look online and see if there are opening/premium seating in those locations in advance. Nothing typically appears that wasn't unsold online beforehand. And premiums especially can convert to discounted seating at the booth, if they held them too long to sell them at regular price.
froote said: "I got front row for Fun Home although obviously that seating wasn't the norm."
Same here.
Otherwise, the closest I believe I've gotten (in recent memory) was Row G for Amélie and Row H for Anastasia (should point out that there were also Row D seats available for the latter, but I took my mom, and she refuses to sit anywhere but end seats). Besides them, I usually get seats about 10-15 rows back in the orchestra, which is fine by me. I've only gotten mezz seats once – Row A in the center for Something Rotten! about five months into its run.
Don't know about A, B, or C but two weeks ago I got fifth row center seats to Hello Dolly with Donna Murphy an hour before curtain on a Saturday night and it was absolutely a perfect experience!
Got front row for Waitress (3 tickets) in February from tkts, loved the show. For CATS, we got row F side Orch, again 3 seats, last October. Great seats for half price and very interactive!