They need heat through there stomachs to aid digestion. If you can achieve that through the use of a basking light and ceramic tile then that of course will work Ok but a heat mat on the deck will provide a constant source of heat.
From the GeckoSpot care sheets
http://www.thegeckospot.com/leocareindex.html
"The heat mat (which I leave on 24 hours per day) enables the gecko to absorb heat through their stomachs, as they would do from sun-heated rocks in the wild (maintenance of the correct temperature is vital to aid food digestion by your gecko). The bulb is efficient at heating the air in the tank, and is also regulated by a timer in order to keep a regular day-light cycle. The bulb goes off at night, providing a night time drop in temperature to the low 70s."
The London Leo Website says the same thing on heat mats
http://www.london-leos.i8.com/custom2.html
I don't want this to turn into another sand as substrate type thread but there does seem to be different thoughts on this. When I was setting my stuff up everything I read on forums, websites and books suggested using a heat mat and bulb combo which I did. All three are fine and happily go from the hot hide to the medium to the cool one.
At the end of the day whatever you've used to heat the viv, bulb or mat, as long as the Geckos are happy and eating well then I don't see the problem but if someone asks for advice then I will suggest using a heat mat as the primary heat source because that's what I've been taught by various breeders and the available information.