I normally run all Picaxe circuits off 3 x AAs (4½ V), so am NOT advocating a move to lower supply voltages. Although not a standard trial by fire (& rarely mentioned to impressionable students),it's almost impossible to destroy a 08M from just 3V. Additionally red/green/amber LEDS can be run without dropper Rs too.
On 3V dead I/O shorts, power supply backwards or even ESD woes etc seem to be tolerated.In fact I've had 08Ms too hot to touch that still perform "OK" when cooler, & have NOT lost a Picaxe since late 2002- in that time working with ~1000s.
Higher supplies (especially 9-12V)naturally MAY be capable of mischief however, & those 9V battery snaps fitted to many Rev.Ed kits make me nervous!
The disadvantage of a 3V supply is of course that some devices just will not function- blue & white LEDs especially, but also IR sensors & 433MHz Rx. Additionally you've less reserve energy on tap with just 2 x AAs, meaning that when they become weary (say 1.2V each) they'll be prematurely discarded. 3 x AAs could fall to below 1V per cell & still keep a light drain Picaxe circuit working. With cheap rechargeables abounding it's appealing to use 3 x NiCd/NIMHs (3.6V) as well, but 2 x 1.2V (2.4 V) is too low a supply.
These points are especially important educationally, as on going battery costs may THE most major expense, meaning a tight budget may be nickel & dimed to an early death. In fact make that "dollar" & dimed, as some AA brands/types (most notably Eveready & 9V)can be excessively costly. Here in NZ I normally use "The Warehouse" alkaline AAs instead, as these cost ~US 20c each (~1/10 the cost of equivalent big name brands), & their life seems to be "ages". Hence I've just switched on a 2003 era Mk.1 PICNIK box,with batteries ~4 yo.,& it's attached "solar motor" <A href='http://www.picaxe.orcon.net.nz/motorpwm.jpg ' Target=_Blank>External Web Link</a> burst into life nicely. Stan