I guess you could pour some coolant down the t-stat hole before you button it up. I never have.
I do have one of these, well worth the $20 I paid for it. It's one of those stupid tools that is not necessary but works great.
I usually fill the rad, fill the overflow to full cold and start it with this funnel on. The air burps, fill more with coolant. Don't over fill it or it can get messy to remove. Just pour in what's needed.. Once it cools down fill the rad, put the cap on and go. If everything is right if it is short it will draw it out of the overflow when it cools down again.