Absolutely fine if you don't want to share, though I've personally never understood this "rule" that we shouldnt discuss money. Why not?

The only reason I can think of is so employers can keep ripping off their workers. In fact, it is EXACTLY how I found out my previous employer was ripping us off. They went BALLISTIC when they found out we knew each others salaries, and tried to make discussing it a disciplinary offence. Telling!

Ask me my annual salary, my rent, whatever. Doesn't bother me at all.