If you put it that way, running any Windows computer consecutively for several days will lower its performance if theres not a hard reset.
Hibernating puts what is in your computer's RAM into your hard disk, and puts it back when you turn your computer back on, so in theory nothing is meant to change after you turn it back on.
Unless your computer is loaded with junk, hibernating shouldn't be too noticable. It might be slower initially when more things are read from the hard disk, but after a little while it should appear all normal.
I usually hibernate my computer, maybe across 3 days before I reboot, not because it stuffs up...usually because I've installed something that needs a reboot or somebody else has used my computer and turned it off instead of hibernating.