A millimeter or two plus or minus may not be a problem. Most frames IMPE are out by that much. But more than that and your rear wheel might not follow the front, making handling a little interesting, especially when you go fast. Why not just straighten the frame?

As far as centering by looking down the valve hole, I don't see how that can be accurate to less than a few mm, if that, and only on the front wheel. That seems more like a "make-do" solution that will get you by until you can use the right tool.

Forgive me for sounding like a snob but I've been a mechanic so long I guess I've forgotten what it's like not to have the right tools and techniques. So, Respect. I admire your can-do attitudes, even if I'm not exactly confident in your workarounds.
I guess if you're building wheels only for yourself, I guess it's one thing. But if you're building for others, that's another. I talked to a mechanic once BITD who told me he would "eyeball dish" (exact quote.) I asked if he flipped the wheel in the stand and he said no. I don't know what technique he used, but I kinda lost respect for him after that.