The only time its really necessary to have precision ground stuff is if your making folders. Then its really important that your getting stock that is a uniform thickness (unless you have the equipment to grind it to thickness yourself)
Otherwise, its not too big a deal. You will want to flatten stuff out as much as possible but your not hurting yourself any, and your saving alot of money.
One thing to keep in mind is that if you just buy the hot rolled annealed stuff, your looking at taking the skin off yourself. ATS 34 gets a black skin/coat of stuff on it when its rolled into barstock that can be hell to remove. There are tricks like soaking it in vinegar for 12 hours and then using a wire wheel on a grinder or angle grinder, but your adding alot of work.
Personally I think its worth the couple extra bucks to buy blanchard ground steel when your using ATS34, and probably anything else like it. Then its reasonably flat, and the skin is all ground off so your ready to get right into the knifemaking. Last time I bought it, I got it from koval for around $18 for .125" X 1.25" X 18" peices. I don't know if anyone else carries it blanchard ground or not.