Lots of very good information here. Thanks to all for replying.
You have not revealed WHICH Arduino board you are asking about.
It's a mega clone from ebay: this one
. It claims to be the reference design but people claim lots of things on ebay so, based on the cautions given by people here, I'm not going to do it.
The way I like to supply regulated external +5vdc to a standard Arduino board is to take a spare USB cable, lop off the PC end, locate the positive and minus power wires and wire them to the supply. That way the built in 500ma thermofuse is in play and it no way interferes with the on board auto-voltage components included the +5vdc regulator chip.
I like this idea. Do you tie the data wires to ground or leave them floating?