When to check English translation while reading?

While reading manga (currently Tengen Toppa Gurren Lagann) I sometimes encounter Japanese sentences that I know all of the words, but don't understand what it's trying to say.

Is it better to check the English translation or just to keep reading? I'm worried that if I always just keep reading, I'll miss out on new grammar or unfamiliar slang.

What are people's experience with this?

by jackbobbins78