I'm thin enough on top that it's a concern, not least because getting sunburned on the top of my head is no fun and having a criss-cross pattern of sunburn seems like even less fun.

I fold a handkerchief and use it to line my helmet when I'm cycling in the sun. Gives me the protection of the helmet while keeping the sun off my head. It's not so great for keeping cool though, by the end of a ride in the summer I can usually wring the sweat out of the handkerchief.