Do you mean that your finger is accidentally touching the very top string? As in the 6th/E string? If it is than I don't really think it's a huge problem since you don't play that string for the C or A chords anyways. You are basically just muting an unused string. The root note for A is the open 5th string and the root for C is the 3rd fret on the 5th string.

Not sure how you would accidentally hit it on an A chord though.