At what percent does it start being guaranteed? This would be good to know for all characters.
I have been looking at doing a post analysis of DB1 -> Utilt on Jibbs, Fox, Falco, Sheik, Peach, Falcon, and Marth. I want to look at it more before simply giving numbers which anyone can do at the moment.
The main point is that DB1 -> Utilt at higher percents is an extra viable option Marth picks up against aerial opponents. If your opponent tries attacking you with an aerial you have the ability to replace say dash back and grab the landing with DB1 instead. The main idea is that the move should work because they made the first move. You simply have to time/space right. If you hit, then at some percent you have enough hitstun to be able to use Utilt and hit them in hitstun.
This does not mean its guaranteed to cover all DI options. As I said its guaranteed in duration of the hitstun. The benefit of hitting with DB1 is that Marth can cover a variety of DI options from simply Utilt or turn -> Utilt. You have ~25 frames on the hit of DB1 to react to the DI and decide if you can cover with Utilt or not. If not, then you cannot be retaliated against for having hit with DB1. Additionally, you still maintain an advantage for simply having hit with DB1 in the first place.
Therefore, its a situation that you set-up which you can end a stock (Utilt KO) or at the very least set-up to follow up with something else if they burn a jump or not.
When the **** are people going to SDI a DB? Be reasonable.
I think the thing you should do foremost is use options that
prevent your opponent from being able to do anything at all if you can. Even if people are unable to SDI/DI DB1 consistently it is a potential option that can occur. Your opponent might simply DI/SDI on accident (or intentionally). Therefore, this is an option you should consider and have a plan on how to deal with it.
Its why DB1 on the grounded targets is not perfect. Yes, you might catch them with no CC at low percents, but every character can CC DB1 until insane percents (above 300%) and punish you for using DB1. In the air, at higher percents DB1 has enough hitstun that characters cannot retaliate against you for hitting with the move. Thus, you lose nothing for hitting with DB1, you can cover several options, and you maintain an advantage.
When I play Puff and am drifting down against a Marth I can SDI up pretty consistently. It's not that hard if you're expecting it. Not sure what kind of answer you were expecting when you ask if something in Melee is "guaranteed". It doesn't even matter if it's guaranteed in a certain situation because it's easier to just react to their DI and attack accordingly than it is to remember character-specific percents, determine which hitbox will be the one to connect, and 10 other miniscule adjustments you will have to make to ensure it's "guaranteed".
I swear, 99% of the time people say "oh, I wish I knew what percents this worked at", they get the answer and continue doing it by their instincts anyway. If I told you DB -> utilt didn't true combo on Puff until 90%, does that mean you wouldn't do it at 85%? I don't understand why you wouldn't since most Puffs don't know any way to deal with it and the ones that do might not be super frame perfect on whatever escape option they choose.
PPMD put this well once before along with Umbreon I believe. Scoping out potential situations and having a plan makes you a better player in the long run. It makes you better at consistency and give you better timing. Mainly since you practice a situation multiple times. Additionally, pressing a button at exactly X milliseconds later is easier to time a move than reacting on surprise.
There is a benefit to practicing CGs on Fox/Falco and recognizing percents. I dropped CGs on Fox all the time thinking I could grab fox in place at like 19% with no DI. I think there are benefits to doing this sort of thing. If the benefit is great enough I think its worth knowing percents. Such as Falcon has options until like 30% off Uthrow. Therefore, I want to tech chase him until 30%, then get easier to land Uthrow follow-ups.
Knowing DB1 -> Utilt percents on aerial opponents is a minor thing. However, its a common enough situation I think and a strong enough position that it can be worth taking note of to minimize times when you use it and get punished because you did it too low of a percent.