I'm in process of building my arcade cabinet. Do you think it's necessary to have fans to keep the system cool? I have built the main body and haven't cut any holes for fans, but I want to make the decision before I start painting.
Nope. You should be fine. I have mine overclocked with no fans. I would only really consider it if you have a large number of internal lights near your pi, or a tv/monitor that gets really hot. Otherwise you should be fine. I keep mine on 24/7.
I put one in because the monitor in the cocktail cabinet was cooking the Pi with the lid shut. With a few wall-wart transformers and the heat output from a monitor on it's back rather than standing up, the heat had nowhere to go. It was just a 1 1/2" 12v computer fan powered from the sound bar power supply on the monitor. and it's enough to keep the temperature pretty cool and push the warm air out of the cab.
It's worth keeping an eye on when you've built your cab to see if it's going to be a problem or not. Might not need fans just vents in the right place to let the heat out and pull cold air in.