A lot of people use 5 seconds and usually do get their bids in on time. I prefer 8. When the auction ends during e-Bay's busiest times, such as on the hour, half hour, or quarter hour, or particularly on Sunday nights, you'll need to leave a longer lead time to get your bid in. During those times e-Bay's circuits get overloaded with so many bids and yours may not get in on time without a longer lead time. The same thing would happen if you sniped it manually.
Sunday nights amount to mass insanity
because of so many auctions ending at or near the same time. For auctions ending on Sunday nights on the hour, half hour, or quarter hour, I usually leave a lead time of 35 to 40 seconds just to be safe. And with anything ending on a Sunday evening I usually leave at least 20 seconds if it's not on the hour, half hour, or quarter hour.
AS's software will attempt to compensate for the busier times by moving your bidding time up if they detect that it may not get there on time due to e-bay's being overloaded, but there is no guarantee it actually will get there on time. Their track record is very good and much better than a person could do manually. So if you put in a bid to go in at, say, 10 seconds and you see AS placed it at 14, it was because their software detected that e-Bay was busier and your bid may not have made it otherwise. It's actually doing you a favor.