Yeah 9 times out of 10 openers have to be approved by the touring act. I do tend to throw what weight I have behind the bands that I like or think will fit the show the best.
It never hurts to just email the booker at the club and ask about opening. You might not get to open for the band you ask about but I do know that emails like that help put a band in context.
Pro Tip: When you do email inquiring about an opening slot please be mindful of what the band you are asking to open for sounds like. I've had bands ask to open for hard rock bands and then turn around the next week and want to open for Brandi Carlile. Moves like that make me think that you don't have your head on straight about your band.