Like the previous post said, it varies. However, I do think it would be safer for people beginning to study to just learn it without using this method so that you don't even need to worry about whether you can use it...
I look at it this way:
Preparing without ruler...
if they don't let you use it, you're OK.
if they let you use it, you're OK.
Preparing with ruler...
if they let you use it, you're good.
if they don't let you use it, you might be screwed.