In most case NiMH should work with a nominal voltage under low load of 1.25 V. This should be just enough to stay above the low bat level. The actual level may vary and some meters may woork OK while others don't. It still depends on the use. Rechargible mainly make sense for really heavy use with plenty of backlight (should be some 10-25 hours of battery life with backlight). Otherwise the Lithium cell are the better choice. With heavy use, so that the batteries don't get old (e.g. < 6 months) even the normal alkaines could be OK.