Some reccomend doing this on high traffic nights (such as Sunday evening)...your thirty seconds may be just 15 in such a case, and a normal amount of time (say, 8 seconds) would not have gone through. Normally I pick 7 seconds or less because I panic and worry about the manual snipers. However, my biggest problems are pretty much your average nibblers, who keep raising my proxy bit by bit...usually the same people too...-_- They don't seem to have caught on to the sniping deal yet. But, there are some snipers out there that are good at what they do, but I have outsmarted them time and again using AS and timing my snipe just right. Strange thing is I know the people that bid on what I bid on since I belong to a message board for what I collect...x_x That REALLY makes things interesting.
Anyway, always check to see when an auction ends. If its a weekend night, ESPECIALLY Sunday, put in some extra seconds for your snipe. You chance it going in too late, although this might not always happen. Sometimes it really will pop up at 30 secs. to the auction's close...but as I said, it's a risk you may have to be willing to take.
*~samuraiblossom~*