6V is probably pushing it. You may not have problems initially, but

could be reducing the life of the internal components. There's no way
to tell by looking for problems. Generally, if you exceed the limits
it will work for a while before failing, and if you exceed the limits
too much it will just be destroyed.

> For peace of mind, adding a voltage regulator would seem to make
> sense.  I plan to try the LM317 that Rich Peet mentioned.

Just keep in mind that a linear regulator wastes power. So, if you
feed 6V into it, and 3V out, roughly half your battery capacity is
being wasted.

You might try making a battery pack that holds 3 AA NiMH batteries.
For longer run time, you could parallel two or more sets of 3 together.

- Adam