There's no need to stretch fiber reinforced packing tape when you put it on -- as long as it isn't loose it'll be fine. In fact, the glass fibers whould probably prevent you from stretching it.

Regarding carbon fiber -- it has its place, but I'm not sure this is it. Try stretching a piece of common 1/2" glass fiber mailing tape to the breaking point: I doubt that either of us can do it. Now consider what would be left of your wing if it were subjected to the force you put on the glass tape trying to break it: that wing would be in the hurt locker, no doubt. So, your wing would be destroyed before it used all the tensile strength of the very cheap glass tape. Now, carbon is also used to add stiffness under compression -- that's because wing failures usually happen when the upward forces get large enough that the upper part of the wing is compressed out of shape and buckles. While it keeps its shape, it's ususally OK, so stiffness is important in extreme stress situations. Putting in a balsa spar (or laying on some balsa sheeting) will also add a bunch of stiffness at very little cost.

Packing tape is great stuff, too. It'll take a lot of tension-type loading as long as the foam (and any spars or other stiffening) will take the compressions that go with it. Packing tape is pretty cheap and it comes in pretty colors. I have a Floyd and an oown-design sloper that have minimal spars and are covered with packing tape and a little redundant fiberglass strapping tape ... check out [link=http://www.geocities.com/sparkir/]this FLOYD site[/link] for more on how other guys use the stuff.

Remeber, engineering is always a balancing act -- the engineer's job isn't just to get the biggest bang out of what he does, it's to get the best bang for the buck. That means we don't want to spend carbon fiber dollars to do something we could do for glass and balsa pennies unless weight is so critical that saving half an ounce is worth the difference in price. (For instance, an advanced competion thermal soaring plane is a good place for exotic materials.) It's not a matter of being cheap, it's matter of conserving resources that can be better used on another project.