Generally if you have not been on a speed restriction and we say nothing we expect the speed limit by the speed limit point for all the reasons you say. I have no problem with peolpe asking unless it is obviously very busy but if we can see a potential benefit of you staying fast we will tell you just as if slowing earlier may reduce you holding we will give you that option.
Some airports however have SLP's a long way further back ie Stanstead from the north, so more often than not the SLP restriction is lifted and "expect 220kts by BKY is more common" hence why on this route the "is there any speed restriction" is very common. Why not move the SLP in that case? Because in the summer or at busy times that SLP helps with other TMA situations.
So basically if we say nothing stick to the SLP's and if it is hectic on the RT dont ask.