Firstly be aware that the KM is almost exactly the same camera as the K1000, and the battery is only used for the metering. The K1000 was the sucessor to the KM and lacked a few of the KM's minor features to save costs. The meter circuits are identical.
LR44 batteries will be fine in a KM/K1000, although the SR44 will be even better - LR44 are alkaline and the SR44 are silver oxide. The meter circuits of these cameras are such as to be not sensitive to the small difference in battery voltage between mercury and alkaline or silver. How long the batteries last for has nothing to do with the meter accuracy. Here is a previous thread on the subject.
k1000 battery ???? - PentaxForums.com
PS : After seeing tonyzoc's post - he is absolutely right and mentions a S76E battery. That is an equivalent battery. Unfortunately different battery manufacturers use different designations for the same thing.