I used to baste with tread, then I used to pin and now I only spray baste. There are several things I prefer with spray basting besides it is a lot faster. I can do extra long twins and Cal King quilts on my own and I like quilting without pins and threads in the way. I just get the $7 can at Walmart. Works better for me then the expensive low filled cans from the LQS. some people use the cheap hairspray from the 99C store and others the elmers Glue Spray can in the craft section at Walmart. since neither of them have the word Quilting on it they are even cheaper and seem to work as well. I have not tried to yet but will once I run out.
As for adding pins after spraying. I generally don't but might add 6 -8 or so (1 in the center and the others on the edges) if I have really large odd quilts like a extra big twin made out of T-shirt material that is ironed onto interfacing. the interfacing does not stick as well to the batting. Also you need a primarily cotton batting I think for the spray method to work but I have used small polly quilts and some one told me wool bat works too. I hope so as I have a Cal King with wool batting waiting to be basted.
Oh one more thing. I spray like ahlf the batting and then wait 1/2-1 minute before putting the fabric down, It seems to stick better this way.