The directions on the bottle say to use Reverse on Day 1 and then again on Day 10 of flowering to prevent male flowers. However, if you look at my signature you'll see a link to a thread I started a couple of grows ago when my White Widows hermied. I sprayed them down on day 25 of flowering and it worked. I've also heard of other growers using it later on in flower with good results - not 100%, but good enough to justify the cost.

I'd give it a shot, honestly. It won't really hurt your plants and it might help. Just make sure you use a wetting agent (DM Penetrator or make your own). And be sure you don't spray it on your actual buds, just on the fan leaves. Oh, and I'd turn your lights off for about half an hour after you spray to avoid light burn caused by the focusing effect of the water droplets. I burned the hell out of some of my leaves when I sprayed my girl lol.