A non cracked case is very important, if cracked invisible moisture will over time tone the strike in a number of bad possible ways.
And when changing strike to a new holder, do so in a dry atmosphere with a qwik short out of case transfer time. This will reduce the silvers exposure to any moisture present in your room. Also do not touch the coin, I always directly do it plastic case to plastic case without actually touching the strikes...
This is a good practice that all strike collectors should adhere to. Newbies take note.
Fingerprints, hazy backgrounds, streaks that show up later will be hopefully averted.