No, don't be afraid - use materials that you don't care much about and just play! :)
What's the worst that can happen? You'll have a quilt with wonky quilting. I have a bunch of them.
Most wonky quilting just about disappears when you wash it the first time - try it with a crib size of whole cloth with a lot of practicing on it. Put it in the washer and dryer just once and see if it's not pretty when it comes out. Launder it a couple more times and it softens up and turns snuggly.
If it's something you want to do, take it as an inner prompting. Don't give up just because you try it a time or two and get thread nests or messy lines. Who cares? You're just playing with scraps! :)
Draw your designs on your practice pieces with markers or stencils or whatever you want. Tape color book pictures to the quilt and sew over the lines of the drawings. Echo around them, doodle in between them.
Don't give up, if it's something you really want! :)