I believe the OP is essentially correct. Often the cheapest fares in a year is in the January-March period, along with November 1-15 and December 1-15.

I would also mention that upgrades are usually easier in January and early December as business travel is usually down then.

So, get a feel for how much travel you would do in any event. Then decide if you want to be platinum or top tier in your program. Back out the mileage you will need to hit it to determine your shortfall.

Then, I look for MRs in the first part of the year to cover 75% of the shortfall. I only get flights that give me EQM at < .05/mile and/or RDM around .02/mile.

Finally, I check where I am around October. If my travel was greater than expected, I either do no late MR or think about going for top tier. This, plus the SEA DEQM offers spurred me to achieve Alaska Airlines 75K instead of Gold.

Other people have their own metrics, and you will certainly have your own. For me, this forces some discipline on how much non-essential time I spend in a tube and not overpaying for benefits that I don't really need or I could buy on an as-needed (or desired) basis.