does he walk on his toes all the time, or just now and again? Is it more often than not?

It's important they they walk on their feet for muscle and tendon development.

When DS does it we tell him to 'use his walking feet', which is the same thing that the staff at his daycare say to all the kids if they are walking on their toes.
Also does he go barefoot or socks only for long periods? My MCHN told me to keep him out of shoes as long as possible as its better for foot development.

You dont have to believe everything your MCHN (or me for that matter, or anyone!) tells you. Its true that if they have any concerns they will tell you, however without you mentioning it to her she had no way of knowing, and if your DS didnt do it in front of her then she'd have no idea - though it does suggest he doesnt do it all the time.

If you are concerned then ring up the MCHN, or go to her, or a different one if you prefer any others in your area - i.e. maybe you have friends that see a different health nurse?