I cut each log to size before sewing it on. I think it is more efficient since I can do all the cutting before even starting the quilt, and I get immediate feedback as to whether my seams are right. If they're not, the logs don't fit and I can correct the problem before it goes any further.
Other hints from Judy Martin (queen of log cabins) - cut so that the log lies on the length of fabric if possible (can't always do that with scraps), and use starch on limp fabrics.