Depends on the establishment. If you're asking about guys' attire, I personally always liked it when a guy would show up in nice jeans and a tucked in button down oxford shirt. The guys I saw wearing that were all European and I thought the style looked great on them.
If you're talking about ladies' attire, I never go to a club without looking dressed up. Even if it's a casual club, that just means I put on a pair of skinny jeans instead of tight black leggings or a dress, and switch out my sexy red top for a more muted color. But it's just my own thing and of course I recognize that I'm a little over the top, lol.
I wouldn't wear jeans and a tee shirt to a club so no to casual. You usually go to the club dressed up. I could see jeans being acceptable for guys ONLY if they're clean and a nice pair, not just some baggy ones you wear to the store. Also they have to be paired with a decent shirt. When I go to the club, I expect the people there to actually show that they cared about looking nice for that night.