Murray - "My guess is that the tonic (I) would be the top note about 75 % of the time, and the dominant (V) would be the top note about 20% of the time, leaving 5 % for the maverick, quirky songs which have one of the other scale notes as the top note."

I tried it with the first ten songs that I sing or play that came to mind and the results are diametrically opposite to your hypothesis!
Try these with what I make the highest note in the song:

Waltzing for Dreamers - V
Please Call Me Baby (Tom Waits) - III
San Diego Serenade - VI
Four Strong Winds - IV
Summer Before the War - II
Rosemarys Sister - VI
Vincent Black Lightning - IV (perhaps bent slightly sharp even!!)
Wild Mountain Thyme - VI
Here There and Everywhere - II
Sisters of Mercy - IV

Not a tonic in sight.