Learning involves making mistakes; don't sweat it.
When bulk rolling, I always do only a few rolls at a time; usually 5-6 or so. The bulk roll stays in the dispenser in the interim. Works fine for me, and the advantages are:
* You build a routine quicker this way as you get to practice a little more often
* A rolling session isn't as long and tedious
* You can correct systematic mistakes
* You can make a small test roll whenever you need it
* You could even get away with just a handful of reusable cassettes
There's something to be said for cutting up the whole bulk roll all at once as well, of course. But the batch-wise fashion has always worked best for me.