Unless you have been riding on really rough roads, you should be fine. I have about 3k miles on my Ultragatorskins and they are beginning to look flat. And as I was changing a flat I noticed a pretty deep slice in the tire. Not sure if it's deep enough to matter. Unless your tires have rotted out from heat, cold heat/cold, you should be fine. Let the air out of the innertube and inspect the tire. I think sidewalls are thinner than the tread, so any damage to sidewalls should be cause for replacing tires. If you go really fast (35+ mph) down steep hills, you don't want to have a tire fail.
Tread on tires is pretty useless. Road tires are slick.