"Needle suggestions" are just that, suggestions. You may knit looser or tighter so need to use a different needle size. What you could do for a dishcloth is edit the pattern to fit the gauge you're getting on the size 10 needles to get the size dishcloth you want. (If you're getting 4 stitches to the inch on the 10's and the dishcloth is 6" wide, cast on 24 to get the desired width.)
Congrats on your progress!