Nice tip!
I've got several different tricks for dealing with valve stems on tubes while mounting tires. There is one trick however, that works well everytime, on every kind of rim and stem orientation.
1. Before ever putting the first tire bead over the edge of the rim, loosely mount the tube on the rim without the tire, inserting and fastening the valve stem with a nut.
2. Leaving the valve stem in place, unwrap the tube from the rim, and pile it loosely on top of the rim, out of the way of the rim edge. This way you know it won't be twisted later.
3. Where the tube stays over the rim edge, gently start the tire over the edge of the rim and tube (valve stem area), and work it all the way 'round. By starting there, you will have plenty of room to go over the portion of tube on the rim without stress.
4. Once the first bead is down into the rim recess, unpack the tube from atop the rim, and lay it in place inside the tire.
5. Continue with second tire bead as usual.
No more fighting with partially-mounted tire sidewalls!
All the best,
Shane