If you don't use a lot of batteries, there's nothing wrong with using
disposables. You may never recoup the cost of the rechargeable batteries and
the recharger.

In my opinion, you should not use rechargeable cells in a calculator (unless
you always carry a backup pair), simply because they might drop dead when
you need them most. Nicad and NiMH cells are particularly bad in this
regard, because they have a flat discharge curve, followed by an abrupt
voltage dive, the worst-possible characteristics for any kind of battery,
especially rechargeables.

Rechargeables make the most sense for medium- to heavy-drain applications,
such as flashlights, motorized toys, and electronic flashes. Calculators are
light-drain, and probably not worth the trouble.