Thanks for the detailed response. Such simple parts of the game get overlooked sometimes.
What about the flip side of that - how do you apply safe pressure with Marth on opponents when they're on a platform above you? Might just be me, but I find I get punished using up tilt. Would fast falling up air and f tilt be safer options?
Also, do you think the back hit of nair might be a good tool to use for this? I don't think I've seen it used much in any situation, let alone as a pressuring tool for opponents on a platform.
BF = Battlefield
DL = Dreamland
DH = Duck Hunt
SH = Short Hop
FF = Fast Fall
Pressuring on a platform with Marth is luckily easier to do compared to most other characters due to his quick start up frame data and him possessing a sword (having disjoint). There are a few things you can use to pressure on a platform.
Utilt is of course going to hit through platforms (of course not the top platforms of BF and DL, or on the left tree on DH, but that's obvious), which can apply pressure. On BF this can sometimes be punished because the slightly lower side platforms compared to DL mean that you'll most cases hit a sourspot on shield. Utilt is still generally the better option in most cases, but Ftilt (the later part of the move, when the sword reaches around 12 o-clock-ish) can tipper through a BF platform which can be a bit safer.
Ftilt won't hit through a DL platform due to the platforms being slightly higher up, but the trade off is that you'll most likely land a tipper Utilt due to the aforementioned platforms being higher up.
Of course you can use your aerials to pressure on platforms; SH Nair is solid, Fair and Bair are pretty good also if you tipper them. However, doing a SH, then using a FF Uair is arguably Marth's best aerial for pressuring platforms. It can lead into lots of things when combined with Utilt, which I'll get into later...
Lets say there's a character on a platform; for the sake of argument lets say Mario is on one of the side BF platforms. Marth is underneath, so he can attempt to pressure Mario. Here you have a few options, such as using your spaced aerials, but one of the best things to do is to condition your opponent. Let me explain...
If in the first instance, you can use Utilt. Mario will be looking to shield this, so if he's quick enough, he might be able to punish you for doing so, with a drop down aerial; Nair, Bair or Uair will likely be the most common options. If he isn't quick enough, you should be able to get another Uair for free to set up a juggle, though let's assume he's confident enough and quick enough to punish.
Then start mixing up with falling Uair in later situations. Mario will now be expecting a move, so he will shield. If you use SH FF Uair, this has less lag on it that Utilt. As such, if Mario will not be able to punish this attempt, so you can then follow up into an Utilt as above. Mario can shield this if he doesn't want to get hit.
Then when you've conditioned your opponent to expect the Utilt after the falling Uair, this is when you can start getting the good stuff. As Mario will be expecting the Utilt after the FF Uair, he will be shielding in anticipation. From this, you can punish by instead using a jumping Shield Breaker. If spaced and timed correctly, you can break a shield after the Uair (assuming the Uair tippered - not sure if it will if the Uair sourspotted), in which case can lead into the stock being taken. Even if you don't break the shield, Mario's shield will now be incredibly low, which allow you to play a little more aggressively until it replenishes.
And actually, it's for this reason why I prefer BF over DL slightly for Marth, because you have to jump more distance to effectively get to the opponent on a DL platform as opposed to a BF platform, and part of the reason why I don't like the idea of the stages being merged in this new suggested ruleset, but hey, at least things are looking like they will change. That, and it's for a different argument I suppose.