i did electrical engineering, but throughout most of it i wasn't really paying attention, so just take the following as rambling.
Some devices have a fixed resistance, like torchlights, and will accomodate a wide range or voltages b4 being damaged. V=IR applies in this case - a lower voltage just gives you less light.
Other complicated devices (like cameras?) have stricter or fixed power requirements to work. P=IV applies. If your voltage drops (significantly) and if your source is able to, a higher current is drawn. More heat in components, possibly shorter lifespan.
Ok, lah, i'm just scaring you. i don't think 7.4V and 7.2V will make much of a diff, provided the 7.2V can deliver the same current, at least. Most rechargeables are rated lower than alkalines - 1.2V against 1.5V, if i recall. Usually works just fine.
[OT: i use an electric toothbrush. i'm *quite* sure the brush is more powerful with normal alkalines than NiMHs. i think bcoz of voltage diff. Anyone using electric toothbrush w similar experience?]