As Rainboe as said, basically the requirements are in there for some reason (usually traffic flow... IE holding arrivals above departures in Sydney).
Some are quite restrictive (130 at Mango, the 9000 on the RIVET arrival into Sydney), and are a huge waste of fuel.
Speed restrictions can sometimes be lifted if allowed by sequencing of traffic (ie if there is no-one in front of you you can speed up) ... Except in the US of course, where its 250 below 10,000 no matter what.
Short answer though... you obey the requirements on the chart unless cleared otherwise.