dd,
Explained my thinking better than I could. Being self inflating, once you open valve, the partially deflated bag does exactly what it's supposed to do.
When I roll my pad for second time, I roll and with both knees, kneel on it forcing as much air out as possible. Once I get the air in last 1/4 of mat, I open valve and continue until the pad is as compressed as I can get it.

DD's observation about the air being sucked back in on the second roll is why I do it that way. I found out that at least for me, I got a much tighter roll than if I opened valve in the beginning and rolled with just my hands.
Your results might be different than mine. Try it both ways and decide what you prefer.