Multiple installs with one database and upgrading

I have a number of WordPress blogs installed using the same database (different table prefixes). They are currently on version 2.7.1 and I am considering upgrading them to 2.9.1.

I know when I’ve upgraded WordPress blogs elsewhere in the past, it mentions that updates are made to the database. Am I correct in assuming that I can upgrade each blog individually without affecting the others? Some blogs are no longer used but I will leave them at their current versions for archiving purposes.