You don't have to upgrade, unless you bid to upgrade and your number comes up. Be sure you have your bid updated the way you want. More than a few pilots have been surprised and have been awarded a seat change they weren't expected and/or did not want.
Very few pilots pass by an upgrade to captain. I did for a few months to make sure I could hold a base I wanted, but I was unusual and I knew it was just a short delay at that time. Second in command time doesn't mean a whole lot for career movement and the vast majority of pilots want pilot in command time ASAP. This is especially true at the regional airlines where first officers make poverty wages.
I have heard of a couple career first officers, though. One guy was wealthy and just flew for fun. He only cared about a good schedule. A few others were forced to remain first officers because of medical problems or check ride busts.
Anyway, it's your choice. Just make sure your ducks are in a row to get awarded what you want.