Will this always work?
Yes. The underscore is all you need, but you can also declare a variable as SYSTEM, and then you don't need the underscore. Just don't get too carried away doing this. All PBP variables need to fit into bank 0 on 14-bit core or bank A on 16-bit core.

Do I have to worry about banks?
Yes.

I only use 18F parts.
Bad news is, (unless all your variables fit into access RAM) you still need to fiddle with bank switching when using assembler. Good news is, you don't need to worry about program memory page nonsense or bank switching for SRF access with 18F parts.