!! Check your local building codes or ask a qualified plumber, check valves are often REQUIRED as part of the meter installation to prevent the possibility of any contamination from your plumbing system to enter the fresh water supply for everyone else !!
If you have mutiple meters manifolded or on various branches of a water main, and there is a large water user in that system, it can draw water backwards through the other meters. Usually only momentarily, unless there is an open outlet on any of the smaller branches in which case it can even draw air in. On digital turbine meters, this reverse flow will be tallied as usage. On Multi-jet and positive displacement mechanical meters, the meter itself will run backwards and deduct from the total on the register, but if it has a pulse output for a digital display or is working with the Next Century system this will create false pulses that add to the total even though the meter is going backwards.
Another reason to always use a check valve when installing a meter is to prevent any downstream contamination from getting drawn through the meter. If you are adding any kind of chlorine, nutrients, or anything other than water downstream, you must make sure that cannot be drawn back into the meter.
One final reason is that it will provide a reliable back pressure and make sure that the meter stays full at all times. This is always recommended to avoid damaging just about all types of meters.
Check valves should be installed near the meter. It can go before or after the meter.
If placing the check valve before the meter, it is important have the proper amount of straight pipe between the meter and the check valve so that flow can straighten and normalize prior to entering the meter. This is typically 10x the pipe size, so a 3/4" meter would require 7.5" of pipe. Yes, the check valve may be attached directly to the inlet of the meter, but that may affect the accuracy slightly on some, so this is not recommended.
If placing the valve after the meter, you can attach it directly to the outlet. Placing the check valve after the meter will ensure that the meter stays full, regardless of what happens downstream.