When you change the timing belt, the water pump & tensioner are staring you in the face. Extra 1/2 hour, if that. If they're ok now, they probably won't last ANOTHER 90k. It sounds like you're not changing your own timing belt, so there's a big labor cost to change the belt. If your water pump starts leaking (or your tensioner pulley starts making noise) in 6 months, you're gonna have to pay the same labor AGAIN because the timing belt has to come off AGAIN to change either of those.

I changed my timing belt & didn't change the water pump. Then within a year my water pump started leaking. I did the work myself, but it was a nice day that I could have spent somewhere else besides changing a water pump. Your car, your money, your choice...