All the stats from EVs and IVs are cut in half at level 50. Fortunately, a 31 IV and full 252 EVs (for a total of 63 stat points, 1 for every 4 EVs) makes 94 total, which is evenly divisible by two, so there's no reason to reduce the EVs.
If for some reason one of the IVs that you're training in is not flawless (Hidden Power or just not a flawless spread), then putting 252 EVs will give you an extra half a point that doesn't do anything, because all partial stats are truncated and not rounded.
tl;dr: With an odd IV, you max out at 252 EVs, with an even IV you max out at 248.