You should absolutely change your M/T trans fluid at 90k miles, and while you're at it, change your rear diff fluid as well. In fact, this is part of Subaru's maintenance guide. In fact, you're probably going to notice slightly better shifting overall with a fluid change.
Here's a link to the service schedule list:
GT Maintenance Schedule
Now... What you DON'T
want to do is FLUSH
your transmission fluid, that's where you can run into problems, and that's where most people run into problems.