Customs help Marth a lot. Way more than most characters.
Dashing assault should be mainly used as a general punish/trapping tool. It's really great.
I use 3123. Reasoning below.
Neutral 3: Dashing Assault
DA gives Marth a much larger threat range. It's great for punishing landings, creating trap situations, roll punishes, just punishing stuff far away. It's amazing. DA just fixes a big problem I have with Marth, and that's his lack of pressure from far away. The opponent has to respect DA. They can't just throw out stuff freely if you've got this. And it's great for forcing people to the edge. SB doesn't do what it does well enough for me to use it, and Storm thrust is just terrible.
Side 1: Dancing Blade
Customs are terrible for side special. DB is just superior and it's a great move besides. It does good damage, it's a great punish, and can lead to landing traps or edguards.
Up 2: Crescent Slash
CS is why you would want customs on for Marth in my opinion. It fixes a huge weakness in him; his lack of options out of throws. With CS, Marth can have scary throw combos that can kill, and do good damage as well. Shields don't just stop Marth with this. It also doesn't hurt his recovery, it just changes it. There are places that you can recover from with DS that you can't with CS, and vice versa. It also can combo out of aerials.
Down 3: Iai Counter
The reduced counter frames are more than made up for by the increased range, damage, and quicker startup. It's just a better counter.
Here's what Emblem Lord had to say about these customs a while ago:
Seems like we all agree on Marth's strongest set.
3123
Neutral B 3 - Dashing Assault
Increases Marths control game in terms of landing traps and what he can actually punish. With DA Marth can consistently bully people to the ledge. The hitbox stays out quite some time and since it's transcendent it will beat virtually any move head on except those with armor or invincibility. This attack is not to be spammed. It ties together his trap game and let's him flow from one trap to another that much more easily.
Side B 1 - Dancing Blade
Can't beat the original. The other Side specials simply aren't as good. DB gives Marth a solid punish option that leads into either a juggle/land trap or ledge trap depending on stage positioning.
Up B 2 - Crescent Slash
This attack nullifies Marth's main weakness. Not having any good confirms or good damage off grabs. With this special any grab can turn into a free Up B for a solid 16% or more depending on if Marth did any grab hits or not. With no rage this can kill at the ledge around 80%. With rage it can kill in the 60's or lower depending on the character Marth is fighting. This special is what moves Marth from lower mid tier to solid high tier. This special also directly enforces Marths strong footsie game. Pokes that force opponents to be respectful and play defensive now fall prey to grab combos with kill potential. This doesnt hurt his recovery so much as it changes it. Now Marth has strong horizontal recovery, but poor vertical recovery instead of vice versa. It does hurt his edge guarding game however. But the trade off is worth it. This attack also vastly increases Marths air trap territory. If Marth has an opponent in the air and the land on a platform with an airdodge, this move will punish them harshly and even kill if they have high enough percent. It can also catch unwary opponents in the air who think they have escaped Marths control when they are attempting to land safely from an air trap situation.
Down B 3 - Iai Counter
It's just a better counter plain and simple. Can aid recovery if he is attacked and even stage spike opponents when used as an edgeguarding tool. Not active for long, but start-up is very fast at 3 frames. Even if it doesn't hit the opponent, the dashing animation gets Marth out of danger pretty consistently making it a huge boon for getting out of bad spots.
