All the arguments about loaded and unloaded voltages are correct. If you measure the votage with with the system on and servos moving, you will see loaded voltage. The loaded meters are easier for this and usually have a go/nogo indication based on the settings of the meter.

My suggestion is to check the batteries with your load meter when you fuel the plane. Make it part of your flight preparation. You should also check linkages, etc. I am a Private Pilot and the FAA requires pilots to use a pre-flight checklist to ensure the plane is ready. RC pilots would be wise to do the same thing. Range checks, landing gear, motor mounts, battery voltages, servo throws should be done a lot more than what I typically see at the field.

A lot of people check everything at home. I do a lot of hangar work too. The transportation of the plane provides opportunities for failures and a field check it best.

Batteries are another issue all together. I have been involved in engineering battery powered equipment for a long time. The real problem with NiCads and battery testers is the discharge curve. These batteries have a very sharp knee. This means that the voltage is relatively constant with only a slight decrease in voltage for most of the charge. When they hit the dicharged level, you are typically done! When we design battery indicators, the low battery level is really unreliable be cause you just do not have enough warning between the low level and the discharged level.

If you really want to know, measure the current used by the plane during normal operation. Normal depends on the kind of flying you do. You can measure the current by placeing an ammeter inline with the battery.

Now look at your battery. It has a mAh rating. This is the amount of current the battery is rated for if discharged in one hour. If you have a 600mAh battery and your current is 125mA for example, you should expect the battery to be good for 4.8 hours. Now for safety, only expect to use 90% of the capacity. 4.32 hours or round down to 4 hours. This is just a for instance. Manage your battery time at the field by keeping the plane turned off when you are not using it. If you don't think you have enough time for a day at the field, you can take a charger or get a bigger (heavier) battery.

You should also have a cycler and cycle your NiCads periodically. This will reduce worries about memory issues. A lot of the chargers have this as built-in feature.

Give your batteries as much attention as you to the engine and the rest of your planes and you probably don't need to spend money on a meter for every plane.

Good Luck.