is there a way to make sure your polka dots are arranged in "pleasing manner". I do not want symmetrical, but want it balanced. Are there any tricks or tips? Thanks!

I have always just used chalkboard chalk to temporarily mark or draw a pattern on a hard surface. For polka dots I would probably start marking a row on the diagonal to any edges of your project. Then continue across the diagonal. When it is all marked you can see the pattern and make any corrections then or as you go. Chalk rubs off easily and can be painted right over the top without an issue.

The dots are going to go on a burlap banner, but the chalk just might work, thanks!

Yep, should work just fine.
We used it on large canvas advertising banners to sketch out designs, before painting them.