I think it really depends on how much time you have to work with them. In the past 3 days I've had 2 that laid their first eggs - on the floor. I went out this morning and found one of them nesting on the floor. I picked her up and put her in the nest box. She scratched around a minute or two, and then jumped out. I picked her up again and put her in the nest box.. She seemed to stay. I went inside the house and came back out about 5 minutes later and caught her on the floor again. I put her BACK into the nest box. She started to get out, and I put my hands in front of her. She finally started scratching about and eventually started nesting. I stayed out there with her about 5 minutes, and it looked like she was going to stay there. So I went back into the house, and when I checked on her in a few minutes and she was still in the nest box. Thank heavens she layed an egg in there!
I'll keep watching her and the other one that laid on the floor for the next few days - and if they nest on the floor, I'll place them in the nest box. It's the weekend and we'll be around, so it's not that big of a deal for me to do that. I work from home, so I can check fairly regularly during the week as well. I realize not everyone is in a situation to keep such a close eye on their hens though. I read a post recently from someone that suggested placing the hen in the nest box with a screen over the front of it and make them stay in there until they lay the egg - but when you see them nesting on the floor, not all day or all night.
ETA: Of our 21 hens, these are the first two that have nested and laid outside the hen boxes. Their hen boxes are about a foot and a half off the ground.
Hope that helps.
Penny