When motivated, or when my hand is tired, I use clutchless upshifts quite a bit. I also do it all rpms. The only thing I do differently is back off the throttle ever-so-slightly to unload the drivetrain before toeing the lever. I honestly cannot feel a difference between using the clutch or not when using this method. I still use the clutch most of the time so I won't wear down the engagement dogs prematurely (or so I've been told this could happen).
You can do downshifts as well, but I normally only do it if really ncessary, like last week when I had a clutch pump problem and only wanted to use the clutch when absolutely ncessary.
When I was a kid, I learned to drive the car without using the clutch, so no problem doing it on bikes. I once had to drive my Z28 through 3 miles of rush-hour traffic with no clutch after the clutch-linkage broke.
stickcar1990, it's fine. Like you said, for upshifts you roll off the throttle for a split second. When you get the feel just right, you won't be able to tell the difference between clutched and non-clutched shifts. There are also a couple of "sweet spots" that feel almost like it "sucks" the shift into the next gear like butter. It helps if you lightly preload the shifter just before rolling off.
You can also clutchless downshift. Be sure to preload the shifter just before you blip (rev match) the throttle. It may take just a little more finesse than upshifts, but if you already learned clutchless upshifting you are already more than halfway there. Most of it is getting the feel. The mechanics of it are the easy part.
I've been doing this for over a decade on and off the track with no ill effect on any of my bikes.
I do both -- up and down -- but more up than down. Especially when accelerating hard with the throttle WFO (blipping at each upshift). Hauling in the clutch lever with the throttle open wastes time and runs the revs up (unless I'm already bouncing off the limiter )...and generally throws off my mojo.
Down is a little trickier, though less so with the slipper clutch. I try not to do it too much though, so as not to overwork the slipper.