I find the range of sizes at my LBS varies noticeably whenever I need to buy them. If they have the size I want, they'll have ungodly long valve stems etc.
Sometimes they have exactly what I want and sometimes I have to "fudge" a bit.
Now, if they have what I want, I buy 2-3 spares. Murphy's Law says IF I have way more than I need, I probably won't need them.