I'd get a thorough check. Trust me. I did a tour last year without such a check and ended up doing about 750KM on two gears (rolling hills). Wasn't fun. I then had to shell out on the road for immediate replacement of cassette and rear derailleur. It's better to have this done in advance of the tour (with a week or so before you do the tour so you can get out whatever kinks you need to get out).
It's worthwhile to pay a few extra up front than shell during. Trust me.