Another reason to not go TOO short is that you may change your mind and put up an 80 or 160m antenna there one day, so having some longer radials will be better and you won't have to do all that work again.
I have my trapped inverted-l for 80/160 10 ft away from the house, so my radial field only goes out 180 degrees. It still works
You always to the best that you can.
I think I read this somewhere, but in those cases the pattern slightly distorts to favor where the radial field is layed out, but not by any good amount. I may have dreamed that, though.