Interesting read:
http://www.motec.com.au/forum/viewtopic.php?f=39&t=769



Quote:

It is reasonable to expect that this method will give the most efficient timing point and provide the most accurate control of fuel mixture but there may be applications where this is not going to give the best performance. The main reason for this is fuel atomisation. If the injector is not particularly good at atomising the fuel better results may be found by injecting on a closed inlet valve. This can also be beneficial for engine starting. This will allow the fuel to atomise better in the inlet port prior to induction but will lead to a less precise control of fuel mixture.

From link above.

Some others say it's best to inject at the peak of air movement, (ie. center the pulse around a point determined by valve max open & piston TDC). My tests seem to indicate early ignition (on closed valve) is better for my Guzzi but it's probably engine dependant.