Penetrate and -DR attributes (as AP bullets have) reduce threshold and DR in general, thats including toughness perks. Therefore i deduce that RBTE works for +crit ressist perks in the same manner.
(BTW helmet (CA mk II) gives +20% crit ressist, not 10%. This all part of the so called sniper nerf)
Edit: If what Tomowolf says is true, then RBTE allso reduces the +10 Crit POWER ressist of helmets by 50%. I have no facts to back this up though.
Not having RBTE reduces the reliability of your build then by this amount (10%, 15%, 20%), because if you dont make a crit you dont do damage. In the end it is a consideration for each player to make.
I can tell you this however, that any powerbuild has RBTE.
I would not take even tougher over LG++, because the third rank is pretty good, and you alredy invested in LG and LG+, and should finish the LG tree.