I lift equipped in IPF. My plan for events goes like this:
I know that the day is long, so I expect that my lifts may take a bit of a hit. Depending on how it's going, I throttle my last attempts up or down.
For bench, I try to find the lightest weight I can touch in with the shirt on, and add a few pounds to that for my opener. No matter what, I know I can get that lift, it's usually about my raw max (my shirt is pretty loose..)
For squats, I keep the knee wraps off for my opener, and use the weight where I go straps up in a training session.
Deads, I lift raw. I take something in about the 80% range, then 90-ish, then go from there.
I'd put the shirt on - especially if this is your first contest. I can't imagine having to put a shirt on between lifts, and not having the proper time to get it seated and practice the groove..