I recently fitted 3 w/tops in a U shaped configuration for No.2 daughter:
Couldn't afford a 1/2" router AND a jig, so went for the router, and mitred the joints.
Nothing was 90 deg.(of course), and I sweated over the job, but all turned out OK.
Woodsmith is 'zackly right with his tips: One from me....If you're not using a jig, rough-cut it with a saw (using masking tape to minimise chip-out), then take very light cuts with the router, down to the line.
And make sure your straight-edge IS!
Pete
PS: One of my 1/2"x50mm long cutters ran-out at the tip, giving a ragged cut, one way only!