I grab when I realize I'm getting shield -> punish a lot. Like, spacing an F-tilt and hitting a shield means that they are expecting a hit from that distance and aren't trying to dodge it, just react or block and then strike back. If I'm aware they are doing it, I go for more grabs. A benefit, if you can call it that, of Samus' grab is that some people will spot dodge to avoid it, but it takes so damn long to come out that you still might grab them after they come out of it. Lastly, people that stick to the ground can be punished by the extender because, as long as you are on top of it, they can't spot dodge it. You can just wait and grab them afterward (but, then again, jumping out of shield completely nulls that strategy, so proceed with caution).
I do not suggest grabbing Puff. What are you going to do out of it? D throw Nair/Dash attack? If you can perfectly time a grab on her, you can perfectly time an Fsmash or Charge blast. Reasonably go for grabs on Peach, because she doesn't have **** for punishment for that kind of stuff if you miss.