Its ok to use it anywhere as long as it's a flat surface. I mean, it does not need to be flat, it just needs to not interrupt the air flow. The thing about the bed is that it will usually "sink" slightly when you place it on the bed, so the air flow that is supposed to happen below it won't happen and it will overheat.
If you put something below like a book or a notepad it should suffice to let the air flow happen. One of those coffee table like this should be very useful as well:
Or even something like this:
That you can use in your bed or in other places.