How straight should you wet sand a painted panel with 600 grit? The ideal answer is as straight as you can, but that's not exactly reality. When wet sanding paint, the goal is to get the texture of the paint as flat as possible. Unfortunately, this is the same time that you will see some of the slight ripples in the panel for the first time, or some of the slight texture that was left in the primer from not sanding it aggressively enough.
You will never fix problems under the paint with clear coat. This means that you will have to follow the shape of the panel when wet sanding the paint....as in, don't use the hardest block you can find. It's tempting to just grab a 1/4" thick piece of acrylic that doesn't flex as all and start knocking the clear down. You will probably end up with worse results. Instead, grab a block that can flex (still acrylic, just thinner) This will follow the panel, and the ripples, and actually sand the clear flat.
It is always surprising how minor of issues show through the paint when you're trying to get it so flat. As always, every single step of the whole body work and paint process is very crucial.
But yes, try very hard to get the paint as flat as possible, but the reality is it's not always possible to get it perfect.