This is honestly, a very, very, difficult question. I think part of it depends on what you need for your team.
Here is what I generally do.
1) If I have a bulky sweeper (Tyranitar, Conkeldurr, and Musharna come to mind), I'll max out hp and the attacking stat it is using, then put the remaining 4 into whatever it's superior defense stat is (and if they are both equal like say Elelektross, then I'll choose based on what is the bigger threat).
2) If I have something that can attack, but its main priority is to either set up status or wall (Donphan and Swampert for ex.), then I'll max out hp along with the defense stat that I'm mostly going to be using, and put leftover into either it's attacking stat, or the other defense stat.
3) If I want a defensive spread about as equal as possible, then I'll put the majority of my ev's into the weaker defensive stat, max out hp, then put the remainder in the stronger defensive stat and boost it with a nature.
4) And then you have special cases. If we use Blissey for example, the general norm is to go with a Bold nature and put the ev's into HP and Defense, so that Blissey can take all kinds of hits (that are not fighting) well. However, there are some that like to milk Blissey's niche as a special wall, and go Calm and put the ev's in her special defense instead.
5) Your spread can also change if you are using stat boosting moves such as Swords Dance, Agility, or Iron Defense, so ev spreads are changed accordingly to accomdate this.
I've tried the 252/128/128 ev spreads before, and they generally aren't that good, since you end up not getting enough of what you really need in both stats. For example, I'll use Leafeon. Leafeon has a base stat spread of 65/110/130/65/60/95. I remember back in the early days of D/P giving Leafeon a 252/128/128 ev spread. 252 in speed, 128 in attack, and 128 in defense. This spread didn't really work for me. While my Leafeon was fast, I often found that it wasn't hitting hard enough to OHKO, and it wasn't taking hits well enough to keep from being 2HKO'ed by things it was neutral to. A superior ev spread would be to put in just enough attack to KO certain threats, then put the rest into defense.
So in short, it depends on what role you want your pokemon to play.
after putting all the necessary EVs in SpA and Speed on my Modest Rotom-W, I'm wondering how it would be best to distribute the remaining 104 EVs for bulk. Both Def and SpD are 250 with 0 EVs in both and a neutral nature to both.
would it be better to just do all 104 EVs in HP or would it be better to split the EVs as 52 Def / 52 SpD?
I'm hoping it's the latter so that it's easier to abuse Pain Split.