No that's not really what you should do. It all depends on the character your fighting ofcoarse but... As soon as your KO'd and still invincable, you should try and get a grab. The best moment to charge the Gaint Punch is after you killed your opponent... Sad but true, you just will not get another oppertunity...
Another option is to SH Charge it. What you do is basically charging the punch in the air, and quickly when you land press Z to cancel the charge. A tricky thing to do is to not fully charge your punch, but not till the maximum. Then you can approuch your opponent with a Short Hopped Charge into a full Gaint Punch... You should try that out.