Guess I am a little confused...What Jenn is saying if you look at the back of your saw, (where the power cord is), and look at the square of the blade to that plate it is probably not a nice 90deg like the gauge says, in fact if you grab that plate and flex it it will most likely even bend...Skill saws are awesome tools for ripping large sheets into manageable pieces and close cuts, but not so much for precision, But your router is and is fully capable of a nice square edge, as you probably already know.
Oh... so you are saying the blade might not be 90 degrees to the surface (not straight up and down)? Whereas the router bits are definitely 90 degrees. Hmmm... I didn't even think of that. I bought a deWalt saw so hopefully it's at least OK out of the box.
Depends on what level of precision you want really (and maybe more importantly your familiarity with your tools.)
I have a pretty decent wormdrive saw that makes very square cuts but the base plate is cast/machined- not stamped.
I get better cuts with that than with the extra step of a router pass myself.
I still check blade angle now and again with a square on all my saws just to get them as close to perfect as possible and that is typically good enough for many projects.
If you are gluing/clamping/nailing/screwing panels together and they will get t-molding, paint, vinyl laminate, etc. anyway then I don't imagine most people will notice anything less than 2-3 degrees out of square.
If doing fine cabinet work with stock that will only get stain for a finish and has drawers and doors...
Then the router method is worth it for sure.
Or as a great alternative to both- a plunge/track saw setup.
A buddy of mine does a lot of nice cabinet building and now that is all he uses anymore really.