Ideally, it is best not to nail, screw or glue the track to a baseboard at all. The reason for this is that plastic and metal, the two ingredients of Scalextric track, expand and contract at different rates. So, on hot days the plastic will expand larger than the rail. Of course, the rail won't let the plastic expand evenly due to the rail gripping the plastic. This causes the track surface to warp.
When the track is laid loose the circuit can 'grow' and absorb any expansion due to heat. If the track is not allowed to move freely (i.e. nailed down) then any expansion will only lead to uneven surfaces.
If the layout is to lay flat, leave the track loose. Keep the layout in a dry and constant temperature room. This normally means that the loft, shed and garage are not ideal places.