AVR-Chat

>
>Then don't connect them that way...   Add a boost converter if you must 
>to cover the low voltage case of 4.4V when the batteries are drained, 
>but it's probably not worth the bother.  NiCd batteries have less 
>internal resistance if you can find them.

NIMH's are fine for this, I'm using 5/4 AF cells at twice that drain with no problems.

Don't charge any nickle based battery in parallel. In theory it's sort of workable, in practice, it dosen't work. 

Maximum volumetric efficiency, is a single cell that fills your battery compartment.

I'd try like hell to avoid any SMPS between the battery and the printhead. 

In some cases, a linear LDO may be more efficient than an SMPS.
Remember, the printhead is only on for a second or two. 

Your static current consumption will be a far bigger impact on battery life, than you think..