It's a personal call. There is no right or wrong way to go about this. If you want a better take on the shop give them a call, tell them you have a product they might be interested in, if they bite talk to them awhile, remember to ask who you're talking to (take notes), are they the owner or buyer (you need to know who to address your line sheet to).
If you're nervous about calling, simply send your line sheet, but send along a hand written note as an introduction.
None of this is a waste of time. If you don't approach shops you'll never know if your product has wide appeal.
About the follow up call, it is very important. Determine how long you think it's going to take the packet to get to the shop, then call within 3-5 days. Always take notes when you call, because a lot of the time they have received your packet but have not had a chance to really look it over, so they might ask you to call back.
With boutiques you are dealing with individuals, not necessarily an "industry."
Just go for it. Make up your own rules.