Recently when i was flying i was descending and the atc told me "fly 280kts or greater" and then i got handed over to APP, at this point i was getting to FL100 which meant that i wanted to slow below 250kts, so i had to ask ATC if I could slow down (which i could).

 

So I'm wondering if I should've slowed down by myself since i got handed over to a new frequency, or if I actually should've asked ATC if I could slow down?

 Being assigned 280kts doesn't alleviate you from any requirements to slow to a regulated speed further down the road. 

In the real world, you would slow down, but you would also query the controller. You usually see this in the climb real world. When traffic is heavy, they will give you speed limit or less depending on the speed of aircraft in front of you. It usually starts with a question from the controller asking "what's your indicated airspeed". Based on your response, he will ask you to maintain current speed or have you slow. Happens a lot in the North East USA. When ready to resume normal speed, I will query asking if he still needs the restriction. Normally once they are done, they will just say "resume normal speed". But sometimes they forget.