okay, being more specific after giving some more thought to what i'm trying to say
i just hit fox with the very tip of nair at low %, he didn't shield. maybe it's my imagination, but i think fox has time even without cc to get out a shine/spotdodge/etc. am i just being stupid and can i just grab him? or should i try to get out delayed knee if he tries anything? and would this be any different playing against a falco?
with CC: it is not guaranteed to get a grab. Opponents can react out of CC'ed n-air pretty easily, but it doesn't make a grab impossible.
without CC: even if its low percentage, the variable percentage differences matter, their normal DI after getting hit, their smash DI, whether you hit with 1st hit of nair in addition to 2nd hit and if they DI the 1st hit, etc etc etc.
80% of the time though its still a conditional grab. grabbing after hitting with tippy toe of n-air though is great if opponent is slow, shields after getting hit.
So I recommend only going for grab vs n00bs and people who for some reason shield a lot after every single hit. It's not like Falcon can hit you quickly after a tippy-toe n-air so a shield as a reaction is kinda weird, but n00bs are sometimes n00bs..
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generally, at low percents if you hit with a weak damage/knockback move it shouldn't be expected for a 'guaranteed' followup such as grab vs fastfollowers ESPECIALLY if they are decent CC'ers.
But it can set up conditioning onto your opponent if they expect you to go for n-air --> grab so in response they spot dodge --> you expect them to spot dodge --> punish
or they can roll/ counter attack out of CC/ jump
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tl;dr, grabbing after tippy toe n-air at low percents virtually not guaranteed, less viable in general the better ur opponent gets, regardless you running at them can cause them to do something that is punishable. Either try to punish them THERE or don't commit too hard after baby hits at low percents
AND this is all assuming u hit with tippy toe, if you hit closer conditions for a grab change, AND assuming spacey you are facing doesn't suck nuts
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a move i been doing more is that when its extremely borderline if I will grab someone, skilled players tend to spotdodge since they are so convinced you are going to grab --> they punish you hard post-spotdodge, a 'safe' punish to this would simply to run-up --> shield --> shieldgrab in response to spotdodge.