How precisely must ATC speed instructions be adhered to?
You don't say what country, but I'll base this on the U.S... There is no "rule" here, however you can expected to be held to the Airmen Certification Standards. These standards are used by your designated pilot examiner (DPE) to determine if you are able to perform according to published standards. 

For example, the Commercial Pilot ACS, depending on what phase of flight you are in, says that you should be able to maintain a specific speed +/- 5 knots. A lot of places in the ACS say "maintain manufacturers recommended speed". 

So the FAA expects you to be able to reasonably maintain an assigned speed given that you were provided your license in accordance with ACS. That being said, if ATC assigns you a speed (N1234 Maintain 200kts until established), you should be OK if you are doing 190kts, or 210kts, but if you are blowing through there at 250kts, ATC will chime in and ask you to slow down.