Anything works, really. T.25-base, T1-base, T5-base, T25-base, T100-base, T500-base, T1000-base, or even T5000-base (or higher, although I haven't personally seen any sets higher than that).
They all have advantages and disadvantages. T25 is most common, T.25 is most efficient (imo), T1 and T5 are often the least expensive to build using casino chips.
One drawback to T5 sets is that a lot of the resulting blind levels sort of dictate that more difficult-to-calculate and/or compile bet and raise amounts are necessary, at least when compared to some of the other base number sets. Counting in 25 increments just comes more naturally than counting in increments of 5. A plus to T5-base is that you don't need to address the funky T500/T1000 chip progression issue, since T1000 chips generally aren't required.
One of the reasons high-roller tourney sets have become more popular in home games is that the higher denomination chips (T1000 and up) in most sets are much nicer than the lower denominations (T25 and lower), especially casino sets -- so you get more of the cool chips in play. Plus you get to sling around hundreds of thousands in bets, making the average joe feel a little more baller like the guys/gals on TV.