If you're filling a near-empty system, you can add liquid. If you're just blindly topping it off, you want to use vapor since the low side carries the vapor portion of the system. If you add liquid, you're it's going to expand and over-pressurize the system. I found that out the hard way a while back with our Civic, damn thing dragged like it had 4 flat tires trying to turn the compressor load, that was until the front seal on the compressor gave way and blew a bunch of oil and refrigerant all over the place...