If you have ever had ich in the old tank, your fish have it! Even if you can't see it, it's there!
TT them all. Easy, sure fire way to make sure no ich makes it into the new tank.
For new fish I use either copper, chloroquine phosphate or TT for ich and then PraziPro for internal parasites. It's so easy to make sure nothing gets in the new tank that doesn't belong.
I think you're right. As much as I hate the extra work, I do want ZERO baddies in the new system. I ought Brute trash barrels, heaters, power heads and air stones to do the TTM. I'll have to do them in batches b/ the trash barrels are only about 40 gallons each.