As others have said, there are a few approaches. You are right to worry about ownership, you probably don't want that. You're better off with the group approach,
chgrp -R www /Users/myusername/subfolder
find /Users/myusername/subfolder -type d | xargs chmod g+rwxs
find /Users/myusername/subfolder -type f | xargs chmod g+rw
The setuid bit is to force the group to be www on anything created under it.
Remember because of the way that UNIX does searches, www group needs r-x on /Users/myusername as well
I question write access. Its usually not good to let it have write access. If this is internal only, ok, but if your webserver gets compromised, normal security is to have it not be able to write anywhere but /tmp/.