I speak from my own experience; naturally, I cannot tell you what you should do but what I think would be the best approach.
Is it common to tell my girlfriend we might go to a strip club in advance? If so, why (or why not)?
In short: yes.
I'm not sure how long you two have been together or how strong the relationship is. Having read your question it appears that you two are in a strong relationship and I see no problem with you telling her in advance.
She will, understandably, be unhappy that your night might result in going to such establishment but it is clear that you have no intentions of doing anything untoward; something I am sure she would realise when you tell her.
How do I inform her then?
Simply tell her that you are going to this Bachelor Party and that the night might take you to a strip club.
They are not ladies of the night, and there is still a bar you can prop yourself against and treat it as the social event that it is.
Ergo, I'd propose you say something along the lines of:
I am going on a Bachelor Party for _____'s wedding. We are going to go to a few of the pubs and clubs around _____, but some of the lads might take us to a strip club. It's not something I'm particularly up for so I'll just stay at the bar until we move on.
I have never been to such an establishment so I have no idea of how it works and what to expect
Imagine a bar with lots of scantily clad women telling you that you are the most handsome person they have ever seen.
Every 5 minutes another woman will come over asking if you'd like to go to a room for a lap dance.
Now, above is only from my experience. I have been twice.
The first is a slightly funny story.
The second was in a similar situation to yourself. I had absolutely no intent on going for such dances as I was in a relationship and did not want this to jeopardise that.
I have absolutely no intention of being unfaithful to my girlfriend
On the next morning, I did tell my girlfriend. I assured her I had done nothing aside from stand at the bar ignoring everyone and drinking alcohol - as is custom ;).
For me, it felt wrong to partake in any of these dances as it felt as though I was cheating. I understand that there are no mutual feelings, but it still felt wrong to me.
The relationship between myself and my girlfriend is very strong. I have known her for 3 years and been together 2.
When I went on a Stag Do (the same principle as a Bachelor Party) I did not tell my girlfriend before. The reason being that I was unsure if we would end up in such an establishment.
The outcome of me telling her is probably quite obvious. She was not very impressed. I imagine she had doubts about my truths. Eventually, she stopped being mad at me, but I am simply telling you this as I am unfamiliar with your relationship and how forgiving your girlfriend may be.
 A surreal experience.
The first time I went to a strip club was with a few friends. I had not long known my, as is now, girlfriend. Then we were simply friends.
At the time I was single; I had one lap dance. During which the girl giving me the dance noticed my poker face. She asked if I was enjoying it and I felt awkward and she could see that.
She asked if I had a girlfriend and various other (very) personal questions about my (sex) life.
She then started an advice session with me telling me that I needed to pluck up the courage to ask out the girl that I liked. It was all very surreal, but the stripper will now be glad to know I am with the said girl.