I don't think there is a set rule on when or when not to use a belt. Everyone is different. And like has been pointed out by other posters, you probably shouldn't over use it.
But on the flip side of the coin, if you aren't sure, don't not use it and risk hurting yourself.
Another consideration is if you have any previous back injuries. Someone with a previous injury my be prudent to use a belt on lifts like BOBR or even "heavy" curls, where someone without a previous injury probably wouldn't need to.
Just my 2 cents.