Actually, the rules say that Psi is not an ability score, and thus can't be increased through attribute increases every four levels. (See the Traveller's Handbook, pg 188, for the specific quote on that.)
However, I'm sure that's ignored as a House Rule in many campaigns.
I enforce it, but I do allow Natural Talent to up a natural 18 to a total of 20, which is the actual topic of this thread.
Hope this helps,
Flynn