In my last question on English L & U SE, I was strongly tempted to write the following:
Every so often I've thought I've chanced across most of them [literary Biblical phrases], but as one keeps reading, one keeps finding more.
but wrote instead:
Every so often I've thought I've chanced across most of them , but as I keep reading, I keep finding more.
My thinking, in preferring the former over the latter, was that while both sentences relate that the first observation is personal, that is, that it was I who thought he'd come across most of these phrases, the latter sentence better emphasized that the second observation is universal; that is, anyone who reads a fair bit will continually discover these phrases in their reading.
Unfortunately, being on a site with an exacting user base caused my inner matronly grammarian to kick in and I changed the sentence, based on some concern I was inappropriately mixing the grammatical persons. As well, I vaguely remember a not insignificant minority of questions on the SAT II Writing as being a stickler for this sort of thing when I took it some years ago. On the other hand, I'm not sure if this isn't perhaps another one of those 6th-grade kludges teachers tell you because either a) the rule is too complicated or b) it's not actually a rule at all, but a myth.
Anyway, sorry for the long-winded background -- can anyone help me and give me a general usage rule, as well as information on how strictly it is adhered to in literary and professional writing?