Is it bad practice to have power lines on a PCB instead of a plane
If you're asking the question, then you should be using traces.

A power plane is only a sensible option when you have particular problems to solve, and have already committed to a multi-layer board.

Use a PCB calculator for your intended finished copper thickness to make sure your traces are wide enough to carry the current you intend.

Your number one priority is to make sure you have a good ground on your board, and that you have a ground conductor under or near to all signal and power conductors, and a path for ground current following the path the signal current takes. A ground doesn't need to be a plane either, especially not on two-layer boards, but often is.