Yes. We have all our R710's fitted one under the other, with no gaps inbetween. We're not running a full rack density (our racks have other things filling the gaps between them like blade chassis etc) but we certainly don't leave any gaps for anything.
It's my belief that in modern racks filled with modern servers in a cool environment you shouldn't need to leave any gaps anywhere except in exceptional circumstances.
As for Dell, we had an Equallogic guy come out to do our initial installation of our first units and he made no comment on it, and he certainly knew about it because he had the fun job of setting up all the SPF+ connects for the 10GbE SAN.