If I'm not mistaken, all games that have been discounted individually by 75% for a day were titles that were released quite recently and that regularly cost more than $9.99 (with the sole exception of FTL which regularly costs $9.99 but only got a 25% discount during the default holiday sale).*
So I think we can expect that most or all games that didn't get a discount at all, or a discount lower than 50%, might get discounted by 50% or 75% for one single day.
All other games (curently at $2.99 and $4.99) probably won't get further discounts unless you buy them in a package with other games from the same publisher or a shared theme.
So if you only want a few different and select games, it should be safe to buy them now at 50% off, unless they still cost more than $4.99 or unless you're ready to spend more on packages to get a better discount on individual games in them (at the risk of buying more than you originally wanted to, even games you're not interested in).
Anyway, just my thoughts, no guarantee. ;)
*EDIT: Hm, I just realized, Trine is another $9.99 game that was individually discounted by 75%. If the regular discount during the holiday sale was 50% already, then my theory is proven wrong.
Post edited December 16, 2012 by Leroux