Sometimes they are too wide. I usually have a selection that are too small I can use spacers on, but I have had to cut them a couple of times.
I use 1" masking tape to create a perfectly straight line around the sleeve. Then I use a Dremel with a reinforced cutting wheel. Slowly go around marking it, before cutting it. I then file off the end with a file.
There are tools you can buy to cut tubes with a hacksaw, but I don't see the point. Let the power tool do the work. I cut bars using the same method too, never gone wrong there either (Oh and I have cut two forks too).