I go with most people in that I tend to purchase two of each deck where possible; one to open and one to keep sealed. However, some decks I have no intention of using (e.g. Bicycle's 'Fashion' series or Flirtatious, Bungalow, Twilight etc) as they are just variations on colour, and those I will be happy with one of.
For actually using cards, I tend to just prefer the most common of the brands - Bicycle for magic and Tally-Ho for flourish (I don't really play cards). I think using strange looking custom cards can put people off. A lot of the time I see card collectors complain that custom decks are using standard faces, but I tend to prefer that. If you're a magician and your cards look bizarre then it can give people the impression that they are being fooled by the deck itself. Of course, there are times when using a non-standard deck can really complement a routine. I'm just quite happy using Bicycle Rider Backs
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This means that although I leave one deck sealed and open another, I never really use the opened deck more than to look at the cards and have a quick play about with them. Then they go back into their box and I go back to my Rider Backs. That being said, if I played around with a deck and thought 'woah, this is awesome' then I'd buy more of that deck, but that doesn't tend to happen. Cards are cards are cards, for the most part. I think one can get bogged down with stocks and finishes when really it's your dexterity that determines how well cards handle.
This also ensures that in the future when I have a wee bairn (or any nosey friends and relatives) who want to see what all these cards look like, there will be an opened deck for them to peruse, and a sealed one to give me peace of mind that I have kept one safe.
Also, modern decks now will not be modern decks in 50 or 100 years.