He is the unquestioned starter. You don't play him much.
We all know what happened last pre-season when he got a bit too cute with the starters and Jake Long got hurt
Keep him in as long as the opposing team has a first team/mix of 1st and 2nd team D in. Want him to get in a groove before the season. He played a decent amount of time last year, and got off to a decent start.
He wasn't any good at the start of last year. The teams we were facing just happened to be worse, mostly owing to Jason Allen.
I dunno, the Bills, Vikings, and Jets all had very good pass defenses last year. 3rd, 10th, 6th respectively. 3 TD, 1 INT isn't blowing me away those first 3 games, but he wasn't turning the ball over and had a 90+ qb rating. Then of course, the wheels fell off in New England. Basically I'm cool with Henne being in there, but let's keep our Longs, Marshalls, and Bess's outta there.