I have been to a few on stag dos, where it is basically compulsory, but never actually paid for a dance, so I have no idea on etiquette. ANyway, certainly, don't feel that you have to pay for any dances. You will get approached by the ladies, they might drape themselves across you and ask you to come along with them for a dance, but you don't have to do anything if you're not up for it. To be fair, it's pretty awkward saying no several times. Being polite seems to work and eventually they will just ignore you. If none of your group is taking dances then you might be asked to leave, unless you are spending money on drinks (which as a rule, seem to be ridiculously expensive).
One point worth noting - don't sit on "Sniffer's Row" unless you're planning to tip paper money. You might get dragged away by a bouncer - saw it happen to a group of lads in a massive place on a stag do in Prague. Ended up in a bit of a scuffle. Bouncers won.
In my opinion, they are awful places. I don't think anyone in the building truly wants to be there. As soon as you walk through the doors you have turned yourself into a fish in a barrel waiting to be shot at. Nothing good can ever come of being in a strip club apart from being able to tell hilarious stories about other people's misfortunes. If anything bad happens to you there, it will always have happened to you in a strip club.