If you're like most Kirby mains that aren't me, you probably use Fsmash quite a bit. To answer your question of "when to use Ftilt," replace those Fsmashes with Ftilts, unless you're going for a KO. It's faster and safer.
As for Dtilt....it depends. You just have to be creative with it. I was playing a good Snake (OFFLINE!!!) on Thursday, and I eventually found myself using Dtilt a lot (against his Wario as well). It's hard to hit Kirby when he's crouching, and the Dtilt has good range, and the trip is great as well. I usually follow up the trips with a Fair or a grab. If you trip your opponent often, and they always react in the same way (i.e. they roll away every time, or getup attack every time) you can punish their predictable reactions with, say, a Grounded Hammer for maximum punishment. Even if you don't know exactly how they'll react, you can just shield next to them and then 'techchase.'
Also, about Grounded Hammer punishing spotdodgers: it actually works, lol. I used Grounded Hammer VERY few times on Thursday, but one of the times I did use it was just running straight up to him and then hammering. He spotdodged, and I hit him and he was like "@#$%"
True story.
Snake is broken.