I've used both methods. When using brass tubes, I make a small access hatch just behind the firewall, which allows me to slip the fuel line onto the tubes. I often make a few hatches in my planes... I don't like having any mechanical stuff or plumbing that I cannot reach or inspect.

On my old trainer, the fuel lines simply pass through a hole in the firewall. The holes are close-fitting without pinching the lines, with the edges slightly beveled to prevent them cutting into the lines. I've found the oil seepage to be minimal over the years I've flown that plane. (of course, you'll want the insides of those holes to be fuel-proofed really well!)