If i understand correctly.....
Headings ending in zero must have the word degrees after them; headings ending in any other number it is optional but not required. This is to avoid confusion between degrees and flight levels when issuing combined instructions or when cockpit workload is high and there could be confusion as to whether the number just given was a heading or a level.
FL100, 200 and so on - the word hundred is used to emphasise the two zeros, as it is easy otherwise to hear "one one zero" or "two two zero" instead of "one zero zero" and so on.
Just a trainee, so if anyone wants to correct me, feel free, but thats how I understand it.