I think doing a buffered dtilt out of a dtilt (i.e. IASA etc etc) is basically always bad. Tipper on hit is a frame advantage for them if they shield drop, if you're then throwing out another move 7 frames later, it's pretty easy for better players to just ... punish you. A reminder that the second dtilt will be pushing you forward.
The 1% is when someone's shielding and you hit them from behind with dtilt. Yes they can buffer a turn around shield and destroy you but that's why you shouldn't ever do it... but at least most of the time you'll get away with that one at most levels of play.
What are you meant to do after the dtilt? Walk. Analyse the situation. Dtilt is meant to hit people at maximum range (preferably of their shield), because Marth controls the range between shield and body really well (max space dtilt on shield into DB will hit a shield dropping character), as a start. Stutter step fsmash for glory cheese.
The thing about dtilt is just the control. Punish their next action.
If their action is to hold shield, then you stand there waiting. A properly animated dtilt won't be pushing you forward (and walking backwards and just turning around negates it), so if you dtilt at the max range again then they're STILL ****ed. If you just dtilt again your second dtilt won't be well spaced, won't be applying the same pressure. Stuff like ftilt and jab (and fsmash and db/etc) are all deceptively safe on maximum range shields and Down tilt is a universal indicator for length that any Marth can feel their spacing with on reaction. So if dtilt is on max range shield and you walk back slightly, turn around again and then jab? They can't shield drop punish that easily because they won't be in range of anything after shield drop (this is why PS dash attack is the core of punishment for characters like Falco). Replace jab with anything.
If they spot dodge, then a dtilt with similar spacing mentalities is COMPLETELY SAFE. Toon Link and Falco can't just dash up smash you if you're throwing dtilts (with timing of course for their end lag) for where they'd be if they moved forwards/where their shield extends to.
If they shield drop and walk out a bit, then you're dealing with a good player. If they shield drop wait, and then shield again for the power shield (which is why double dtilt is bad against anyone comfortable with that timing) you're going to die, and are probably dealing with a really good player. Make sure you tell them they're not being sneaky and hopefully they'll get scared. Alternatively get better (moving forward yourself, dancing blade, dash grab, etc etc)
If they roll away, or have the tendency to roll away, then you react to shield drop with the assumption of rolling and just input your dash punish (or walk punish if possible). The cool thing about properly reacting to rolls is getting the direction wrong shouldn't be bad for you (bar ground room issues). If you think its a roll behind you, then instant standing pivot grabs is cool, or just walk a bit and turn around dtilt (if you're comfortable with knowing their roll lengths you can just adjust to pull this off).
People jumping are asking to die. Ftilt.
What about the other situations? OoS options or moves that are really fast (jabs or dash attacks). Character specific. As I assure you though, a well spaced dtilt isn't easy to OoS punish. Characters or players prone to try are asking for you to shield (preferably power shield it), or the "lazy way" (read: what mikeneko and ramin and everyone else including you and me do) is the immediate spot dodge. We're getting away with this and lets not pretend that people won't figure it out at some point.
