Can't hurt to turn mouse and keyboard off when not in use. You didn't mention which keyboard/mouse combo you have. Are you sure there's no status LED on the keyboard? If it's Dell's KM636 Keyboard+Mouse combo kit, this doc says there's a keyboard LED which flashes orange when batteries are low.  I presume that's the same LED which turns white for ~15 sec when the KM636 keyboard is first turned on, and then automatically turns off. That doc also says you may notice some keys stop working when the batteries are low, typically C, E, and Esc stop working... If you have a different keyboard/mouse combo, there should have been an instructions guide in the package. Otherwise, search for whichever keyboard/mouse combo you have on Dell's support site.
I have a Dell Inspiron 5400 which I bought last January. A yellow light on the wireless mouse started blinking indicating that the battery is running low. I replaced the battery and now it works fine.  Both the mouse and the wireless keyboard have batteries. Under the mouse and under the keyboard there is a power switch which turns the keyboard and the mouse off.  1)I use the computer every day but when it is not in use should I turn the power switch off?  2)When the battery on the keyboard is running low how is it indicated? There is not a light on the keyboard so I'm wondering how could one tell if the battery in the keyboard is low. 
Battery is running low