To charge for that info is bad policy but to withhold it in certain situations is okay. Once you set a policy of charging for petty services, you'll have to charge everyone equally which will certainly turn off (enrage) good customers. You can, however, withhold that info from people you judge not to be potential buyers. In the long run though, it probably doesn't pay. For once you've shown your expertise to people who take your measurements and run home to order off of the internet, those same people might be more inclined to buy their next bike from "the guys who know what they're doing."