One word: PLACEBO
Adding a rubberband does not increase the tension even by 1 lb, much less 5 lbs. The very idea is ridiculous, so let me just stop it now before any more people get confused about this. A rubberband is virtually identical to any other vibration dampener, and the tension effect is negligible.
For those who still don't believe me, try this experiment. Get a tension meter, and measure a few points in the middle of the stringbed. (It might say something like 45 lbs for a 60 lb stringjob. This is ok.) Next, add 10 rubberbands around the edges of the stringbed. Then use the tension meter to measure stringbed stiffness again. Make sure to measure the same points on the stringbed as before, as the consistency is lower than you would guess. The reading will be exactly the same. It will not read 5 lbs greater, much less 50 lbs greater.
I hope at this point the absurdity has become apparent.