Leder,
If ATC are doing their thing correctly(!), if there is no speed control instruction given to you then fly the speed you want to fly. If you are given a speed restriction ie 160 kts until 4 miles then you must fly at that speed or else you will erode the spacing required by ATC.
If you are constantly being asked to expedite clearing the runway then this sounds to me like an ATC problem (unless you are one of those companies who are always very slow to vacate!). If they slow you down so much that they always have to ask you to expedite, then they should have slowed the one behind you down too. Also, although you do have ultimate responsibility for the safety of your aircraft and must ask if you think that you don't have the required spacing for either separation or wake turbulence don't try to 'second guess' what spacing ATC are trying to achieve. The gaps at any airport can vary significantly from minute to minute depending on the number of arrivals, departures, runway inspections etc etc. As an Approach Controller having 3 or 4 pilots in a row all trying to help out by adjusting the spacing to what they think ATC want would be a nightmare!!
This is just my personal opinion, I'm sure others may take a different view!
Regards, ADIS