There's nothing wrong with putting thought and planning into your battery needs, but at the same time don't lose any sleep over the subject. The good news is that you have a Sony m10 and they last a looong time on a single charge...whatever battery you choose. For example, I think people get over 20 hours on a pair of alkaline's. Just watch the battery meter and change your batteries out before it goes dead on its own (I can't remember if the m10 saves the file if it loses power or loses the file, but just good practice to hit stop and change the batteries before the recorder goes dead).
I'd add to Brians response that Eneloop batteries are low self discharge, so they're very popular batteries in the taping circles. But really there's no bad rechargeable battery in terms of 'don't use this' type of thing. Most people use the 2500mah Energizer rechargeables that you find at Wal Mart without any real down-side.
One thing to realize about battery discharge and the meters is that the meters show bars based on the voltage reading. The discharge voltage vs. time is NOT a linear relationship, so you might see 2 or 3 bars for hours and hours on the M10 but the amount of time between it going from one bar to going dead might not be very long (I don't have an M10, so I don't know how long that time might be). Pay attention to the meter though and don't assume that just because it took say 2 or 3 hours to go from 2 bars to 1 bar that it will also take that long to go from 1 bars to zero bars.