When to add second box after getting nucs

Raleigh, NC

Hey there,

I'll be getting my nucs soon and adding them to 8-frame boxes. Would it be a bad idea to immediately add a second box to the hives once I set up the bees, or should I definitely wait until they've filled up 6-7 frames? I'm just thinking about how likely they might be to swarm and I'm trying to figure out how bad of an idea it would be to add a second box when they're taking up 5 frames vs 6/7 frames.