I usually buy something at the lbs when I stop in, but not always. Typically if I ask them for advice I'll buy something from them. The most recent time I didn't but that was because it was a "bring your bike back in and we'll see what size you need" kind of thing (a FD shim that I knew what size I wanted but they weren't sure whether the numbers on the part were for the size of tubing or size it'd shim it to with the shims they had laying around) and I haven't had time to bring the bike back in yet.