In message , The Medway
Handyman writes
Do NiMh batteries self discharge if left unused for a period of time,

Yes, fairly quickly as well - can be noticeable in a matter of weeks?.
Though the newer type with a lower self discharge rate really do hold
charge a lot better. (Sanyo eneloop, Uniross Hybrio etc.) which is why
they can sell them ready charged.

or is
this just a myth?

No.

Except for thing like the digital cameras, where we tend to cycle
batteries pretty frequently anyway. I'm finding that I'm moving over to
the lower self discharge rate ones when I get new ones. for general use
I find them more convenient and they last a decent amount of time in low
current drain applications.
--
Chris French