I do not spray baste. Most of my quilts are either small enough where I can pull both the top and bottom taught when pinning, or big enough where I have to lay it on the floor. In cases of bed quilts, I feel like I wouldn't be able to get the quilt all square. I lay on it, and start pinning from the center out. I secure the backing first, then layer the batting, then put the top on. Then I make sure the quilt top is taught going out from the center, and usually the back is fine.
For the smaller projects, like I said, I just make sure the top and back are taught at the same time, working out from the center. Haven't had any problems doing it that way, though I will say I wish I had a rack for medium-sized lap quilts. I don't think I'll try spray basting, but that's just me. It seems like an unnecessary chemical. Safety pins for me!