The get slightly stronger. If you plan on level gaining a bit(do it at the beginning of the encephelon...best amount of TP,EP,SP to EXP ratio...also killing all the enemies at the same time on a x2 or x4 gets you lots of everything) then its up to you. Upgrading a tech is good but so is upgrading your character stats(you can only go as high as the highest mark in the group...so if your vitality stats are 55,54,56,57,55 and 60 then no one can surpass 60. Its their way of making it so you can't over do it).

Its really up to you and what you decide the pay off is. I don't really upgrade my techs too much personally. I do the speed and recovery time though.