Dyson is probably the final word, esp. if you're using him, but here's my philosophy:
Use Terry to do your analysis if you can afford it, and get every sample analyzed.
If you can't or would rather not pay his prices:
You can get analysis for $10/pop if you use Schaeffer's. So do it every time. $40/yr or less than $4 per month. And you get to post it on here and add to the collective wisdom of the board. Of course, for a little more you could use someone who does more information, etc.
It is very little hassle if you get a bundle of prepaid bottles. It will give you an early heads up if you develop a filtration problem or head gasket leak.
If you don't do your own changes, any sane garage will be glad to pull the same during your oil change.