Hogdogs has it right regarding accuracy with your gun. Distance in general will never have a substitute.
However, if you're talking about honing YOUR skills, then I think it's not only acceptable, it's a great idea.
If you ever attend Appleseed, they start out simulating targets at (500yds?) using a target at approx. (83?) yards. The target is the size of a postage stamp...yes, a postage stamp. My numbers are probably off, but you get the idea.
My take is by all means, practice safely using smaller targets as you're wanting to do...