However, for DIY etching, leaving any large sections of extra copper in place, both sides, is a reasonable practice. First, your etching solution is going to last longer because there's less copper to dissolve. And the process will be a bit faster, because your etching solution remains fresh longer with less copper to dissolve.
I haven't done it in ages. For small boards, it's pretty cheap to send them out these days. And if I need something tomorrow, I'll just do a one-off with a proto board.