If you are topping off a system, the only choice you have is by pressure. If you are following the factory service manual and all you have are gauges, then following the manual with gauges will get the job done.

If you have a charging scale, and you are charging an empty, evacuated system, charging by weight is very simple and foolproof.

I only recently bought a charging scale and have found it to be REALLY handy. It is beyond handy when charging a system converted to R134a because you can charge to the 70% level by volume, then go up in increments of 1 oz. This is the very best way to deal with the dreaded 134 conversions.

All that said, I have been charging a/c systems of all types with nothing more than a set of guages for about 45 years, so a scale is definitely not the only way to get the job done.