If you've ever seen a truly taco'ed wheel it's far more than just out of true. WHen a wheel tacos it actually snaps past the spokes on one side so the spokes go past vertical and no longer support the rim properly with one side in opposition to the other. If you don't actually snap the rim past vertical on the one series of spokes I'd suggest that it's not taco'ed and is just "out of true".
http://www.instructables.com/files/d...7HZ.MEDIUM.jpg
Now THAT is a taco. Note how the spokes from one side are no longer under tension and are actually sticking up out of the rim? When the guy stomps it back down it'll drive those loose spokes back over center and they'll then have their tension restored. It won't be pretty but if it doesn't explode it'll get the guy home.
Tensionometer is a nice thing to have but you don't NEED one to build or true up wheels. A lot of us do it by ear through setting up the spokes to even musical notes of the desired value. I've done a good 12 pairs over the years and only had minor tuning issues with one road wheel and no more than the issues I'd expect with my mountain bike wheels. All without a tensionometer. I'm sure you'll find a lot of wheelbuilders that built hundreds of wheels in the past without them as well.