My only point is that a prime numbered cog will allow any number of teeth in the front and maintain the maximum number of skid spots. But yes, if you change the gear ratio primarily by changing the cog in the back, you can put in a 43 or 47 tooth chainring in front and use any number of teeth in the back. I usually change the chainring because one can adjust the gear ratio more finely by changing the chainring by a tooth or two. (edit: Oh, and because with only a chainwhip and my lack of cog changing skill, it is much easier to change a chainring if I want a new ratio.)

I just said "continuous" because, in practice, 17 wear patches becomes a continuous strip around the tire.