But only if you're more than 5 short on the placebo epic power and cost, which unlock at 72, with the first mission for the next placebo tier at 71. At this point it should be taking you about a week a rank, and keeping an eye on being more than 5 cards short 6 ranks ahead, will allow you to take advantage of buying at least one epic capsule.

If you're less than five cards short for epics, than you're fine to wait until the rank before they're needed, or even the rank that they're needed. Every epic card should come up in the shop about once a week.

It's best to have both the epic power and cost, though you really only need one. Epic power is far better. Epic cost allows you to buy 10x as much, and allows you to start buying them sooner.

Otherwise I definitely agree with the prevailing wisdom of waiting until the endgame to jump on buying every epic capsule.