For the vast majority of characters, you shouldn't have to worry about staling a kill move. You either have A) plenty of ways to refresh said move
or
B) other good kill moves
Now, there are times (like when your opponent is at 110%) that grabbing is better, because you can pummel a lot, and still get a throw. However, the entire concept that, "a throw can be followed up better" is a really stupid argument. When you smash someone, they either
A) End up OFF STAGE, which is the most advantageous position a player can gain in this game
or
B) End up in the air, and juggle or landing traps are almost every bit as effective as most advantageous positions a character can possess out of a throw.
OBVIOUSLY, there will always be times when grabbing is good too. If you are King D3, then you should probably grab characters you can chain throw whenever you have a good change, plus your bthrow does 16%. If you are Falco, and your opponent is open and at low percents, you want to get in that chain throw to combos. With Marth, fthrow 0 I can fthrow to Fsmash a lot of characters. However, in many, many instances, there are times when your opponent is open, and you opt for a grab, instead of a smash attack, simply because it's the universal option. Some characters don't even have amazing throws, and most times, a follow up out of a grab is predictable, and doesn't always work. In this game, RELIABILITY is important, so when you can land a smash for a reliable 20%, I advise you do so.