Should I store CDKEYs from other websitein a file or only show key when I need to claim/trade?

I write all of my keys in a notepad and save it in my files :)
I would recommend that to anyone, and not just with humble bundle keys, everything.

If you have a significant amount of keys that you're worried about I would definitely use Google Sheets to store and organize them, because it's free and you will have a secure copy of the file backed up in case something happens to your local storage. I use Excel because I usually get rid of my humble leftovers before the next month so I only have a dozen or so at a time, but if I was worried about any high-value keys I would use Google Sheets.

As a Data Analyst this is actually something that I ask people sometimes when trading. I do cringe when people say they use a basic text file, but hey it gets the job done. One person gave me an invalid key and they said "oh sorry, typo" and I asked what do you mean, you're typing this key manually? He said yeah, he takes SCREENSHOTS of the keys and saves the images, then apparently re-types then when he needs them... that blew my mind...

I just use my keys right away, either I activate them myself, give them to friends that want them or I give here in case friends don't want them either, within a few days of buying all keys should be gone so no problems here.

You should us the keys asap, because if anything fails on the chain you will lose your key and the possibility to claim it. If you see keys that are given away for free on sites like HumbleBundle, Alienwarearena and similar, most of the times they say that you should claim it before X day (usually 2 or 1 week) because after that they might delete unused keys or make it useless to avoid re-selling.

Excel sheet in a column next to my game list. But tend to not keep them for months, due risk of keys getting pulled.
Still happens anyway i found out 2 days ago.

HB is really sick with this banning/cancelling