Yes, by all means buy and 'allocate' the cards with a post-it, but don't write the actual card months in advance.
Aside from new babies who don't get a mention, and deceased family members who do, any small things could have changed in their family circumstances in the meantime.
Just as two of many possibilities, what if you'd bought them one of those 'humorous' cards based around drinking lots of alcohol on your birthday, then one of them had since come out as an alcoholic; or worse, had known somebody close who was killed or injured by a drunk driver? Or if you'd got them a 'ooh, you're so old, you probably can't remember how many candles there are on your cake!' 'funny' cards and, in the meantime, they've been diagnosed with dementia?
Also, you can plan ahead for birthdays, anniversaries, Christmas and the like; but you still have to take notice and act quickly and accordingly for cards for bereavements or serious illness and similar.
In fact, you could find that, once you get a reputation for being hot on promptly sending over the cards for all the predictable annual events, it could work against you and make you look even worse and uncaring by contrast if you then drop the ball for a card for a non-predictable event. It might look like you've ignored or snubbed them terribly, whereas for Uncle Kevin who never remembers any cards, "Well, that's just him, isn't it - he's a bit hopeless but he means well!"
Also, I presume our town isn't unique in seeing the 'last collection' at all/most of the post boxes brought forward to 9am, so don't get caught out there. Effectively, first-class post has now slowed down to a minimum of two days, as you have to post it the night before/very first thing to make sure that it's collected and then hope your posties don't just stuff them through any old door at the other end, as ours frequently do.