It doesn't matter how you prefer to wire them. You must wire them in a way that matches the devices to the power supply.
If they are "dumb" resistor-limited units, and they are an identical match, then it's easy - hook 2 in series to a 24V supply instead of 12V. Or 3 in series to 36V, but that's the end of the road - any higher and you lose the low-voltage exemption, and you need to treat DC with the kind of fear and respect you ought to treat 10 times the AC voltage. It's a nasty customer. Do not mess with that unless you really know what you're doing.
If they have an internal DC-DC converter or constant-current regulator, then you cannot wire them in series. You must wire them in parallel at any of their spec voltages.
If you are driving bare LED emitter components, such as those made by Cree, where the spec sheet saysto drive them at a constant current... Then series connection is the preferred way to do it, since one CC supply can drive several emitters.
If you are hooking up LED products in series because you read the last paragraph somewhere, then read more :)