I agree with Ken Shaw who wrote this at woodwind.org:
It's totally legal, moral and non-fattening to slide. German system players slide between clarion C and Eb for a living. A little nose oil helps. Get some lubrication by rubbing your right pinky over the area where your nose meets your face.
In fact, sliding should be something you practice. Watch a bassoonist's left thumb for a while and you'll realize that clarinetists have it easy.
Thanks to Gary Kern for sending that quote!