--When using multiple supplies in series, connections to the ground lug should be made on only one of the supplies to prevent ground loops.
When putting multiple supplies in series, it only makes sense to connect one of the terminals on one of the supplies to ground, otherwise you would be shorting some of the power supplies.
--I am pretty much of a beginner myself, so please correct me if I am wrong. And, I still have very little idea when or when not to float a supply.
Grounding is often fine and safer in some cases. If you connect a floating +5 VDC supply to mains, you will have a nice 115 V / 230 V AC signal superimposed on your power supply, as opposed to a tripped breaker as you might expect. Grounding at multiple points (supposedly at the same potential) creates loops, which can induce noise, so floating supplies may make life easier when working with low level signals. Floating supplies can also be nice when working with a scope, since you can connect the ground clip anywhere in the circuit. Keep in mind that the ground clip of the other channel is connected to the first one, however.
In many cases, it doesn't really matter, but to ground it by default is slightly safer in my opinion.
I have to laugh -- my el-cheapo wun hung lo PSU came with a two-pin mains plug -- ie: no earth!
Does it have a ground output terminal? That would be funny. Apart from that I don't see why it would be unsafe provided they designed it as double insulated, something that they might have skipped.