How perfect do I need to be when planing?

I’m building a box with rabbet joints at the corner with a lid (8x14x24”) but I had to joint two boards together to get the width I need. How flat does the face of the joined board need to be? I’ve got a couple small gaps (0.012”), but it feels like I’m just chasing my tail at this point. Mostly using a no5 with a no4 for