Getting 5VDC from 6V (4S 1.5V AA alkaline batteries)
(Question subsequently fixed) First off, to get 6 Volts from the alkaline AA cells, they would need to be wired in series, not in parallel. Paralleling batteries will increase current delivery capacity but keep the voltage the same as a single cell.

While pre-built buck regulator modules are available at the same source (eBay.com) as the boost regulator mentioned in the question, and at a similar price, the following are the trade-offs:


These buck regulator modules have a measurably higher efficiency in practice than the boost regulator linked in the question - The boost module gets quite warm (I use that specific module for charging my cellphone from a single AA cell while traveling)
The boost regulator is significantly smaller in size 
The boost regulator works until the batteries are far more depleted, than any boost regulator strategy would allow.


Hence, going by the last two points, the boost regulator module is certainly a preferable approach, if the device running a bit warm is not a concern.