the problem is that with the reduced amount of water that modern machines use they will be pretty much full of cold water by the time any hot comes through to the machine from a hot pipe, although it may be cheaper overall to heat water by gas with a washing machine you are heating the water twice, once by gas, and once again by electric after it's filled the machine.
If you can get rid of one of these heating stages it going to be cheaper, ideally you would have a couple of feet of well insulated pipes from the hot water tank to the WM so it gets hot water as soon as it starts filling, but this is not normally practical, so the easiest way to get rid of one of the heating stages is to just fill it with cold water.
The problem is no better with combi boilers as they generally take a couple of minutes for hot water to start running after a tap has been turned on and for this time you are burning gas for no benefit to the heat of the water going into the machine (and again all you end up doing is filling pipes with hot water so it can just cool down)
if you could time your washing machine with any other activity that will run off the cold water in the pipes (i.e. doiing your washing up) then hot fill would be better as the machine would get hot water pretty much straight away.