Meh - I'm by nature fairly cautious. The only place I live on the edge is in using fabric without washing. Never had a problem. If I'm concerned I wash with color catchers, and have had them come out pretty well dyed! They did the job, though - no bleeding in unwanted areas of the quilt.
As a very general rule, I don't use pre-cuts because they aren't usually a good value. Once in a while, though, I find a great deal and grab it. A jelly roll of 40 strips = 100" of fabric, all cut into strips and ready to go. If you get it on sale for $20 (not unusual at Missouri Star, or sometimes other places), you are getting just under 2.8 yards @ about $7.14/yd. The same is true for a layer cake. Depending on what you are looking for, or if you want to be able to use the whole line, precuts purchased on sale can actually be a good deal. Add in saved time, and can't be beat!
Right now, I'm working on a guild mystery quilt that required a jelly roll OR 42 2.5" strips we cut ourselves. I'm using a jelly roll I bought on sale that was in my cabinet.
So while I don't generally buy them ... I don't have any rules other than to enjoy what I'm doing! I prefer the crinkly look that unwashed fabrics have after a quilt is done, quilted and washed at that point; I wouldn't get that if I prewashed, plus I've saved time.