This tip has never made even the slightest bit of sense to me. Unless you're someone who struggles to complete event prize tracks until the last possible second, this tip is actually the exact opposite of what you should be doing (and even for those who do struggle to complete event prize tracks, it may still be the wrong decision).
Saving up to three daily challenges for any given event will, at most, give you a head start that's equivalent to about one day's event currency total goal (assuming you earn the same amount each day by logging in the same number of times each day - which is how all of the model calendars are based).
In contrast, doing all of the "normal" daily challenges up until the event starts gives you three additional opportunities to earn donuts from daily challenges, which can be up to 6 donuts per day or 18 donuts total over 3 days. The only way that getting extra event currency will help you earn more donuts is through bonus rounds, and 3 extra daily challenges are never worth more than, at most, 1 bonus round (which means, at most, 3 donuts). It doesn't make any sense to throw away a chance at 18 donuts for a chance at 3 donuts. Also, keep in mind that there's always an option to buy the last prize if you realize you won't make it in time. Even in that case, if you're actually close enough that having 3 extra daily challenges would matter, then the cost to buy the final prize will be somewhere around 0-10 donuts total (which, again, is less than 18).
So, unless you don't get or can't switch to a donut-earning challenge before the event starts, this idea of saving challenges to turn them into event currency is actually the exact OPPOSITE of what you should do (except possibly for those very, very few people who will miss the final prize by just a couple of collection cycles and who somehow can predict in advance that they're better off throwing away guaranteed donuts before the event to reduce the amount they'll have to spend at the end of the event for some final prize they just have to have but couldn't earn in time).
If anyone needs further explanation, I'm happy to give it because it drives me crazy to see posts like this before every event when simple math dictates that this "pro tip" is completely the wrong solution for the vast majority of players. (And if you want to argue that the head start gives you peace of mind that's worth more than the donuts you're throwing away by following this "pro tip," I get your argument, but it's based on emotion, not a purely rational choice.)