Google typically advises you place them off the ground and in plain view, like on a shelf or TV stand in a central location at eye level. That's not to say you can't do what you're suggesting, I've certainly seen some users ceiling mount them, just that your mileage is going to vary here.
It's worth bearing in mind you want your primary Wifi point to be as close as possible to the centre of your house. This is because Nest Wifi uses a fewest hops approach. If one of your Wifi points can get even a weak connection to your primary Wifi point, it will do that and will not go through another intermediary Wifi point, which typically results in poor performance. I don't know the layout of your roof space but that is to say that you shouldn't place them in a straight line with your primary Wifi point at one end and not in the middle.
Honestly, hardwiring is a much better way to improve performance. The main thing to note with this is that your other Wifi points need to be connected downstream of your primary Wifi point.
Hardwire Wifi devices with Ethernet - Google Nest Help
If you choose to go this route, not sure which Wifi model you're looking at but good to know that Nest Wifi points don't have ethernet ports and can't be hardwired. That's the only model that can't be hardwired.
