It is not normal for a trans to slip at all, for first start or after its been parked for a while, throw trans in neutral while car warms up, the torque convert drains when parked and the pump doesn't move much, if any, fluid in park, but pumps in neutral and gets torque converter filled before you put it in gear.
Yes, I've started doing this as I realized that letting it slip into reverse obviously was a bad thing. It helps quite a bit. I had gotten to used to vehicles that warm up in park.
