Not at all!
'Grab' is a very common everyday word, meaning to to take in a rough, careless way.
'Snaffle' is a very unusual colloquial word, meaning to take something for yourself so that another person might want.
All native speakers use the word 'grab' in many situations, whereas I would guess that 90% of native speakers have never used the word 'snaffle' in their whole lives.
As Peachey says, you would sound very funny indeed if you used 'snaffle' as a substitute for 'grab'.