Wow Logiatrix! I think it's wonderful that you have your grandmother's cards to read with! I purchased a deck of cards - bicycle like you, to read with.
I have read in cartomancy books, (don't know which one(s) they are today), that it's fine to also use these cards to play regular card games with too. I actually like a deck that is worn as it feels 'warm' to me and more inviting. I do like my playing cards, and these were the first cards I learned to read with, but at times I do find myself more drawn to the Rider Waite deck, (my 2nd deck for reading, but 1st tarot deck).
While, I probably 'prefer' playing cards, (seem to be more mystical to me, and at times more accurate), I just allow my whims to choose the deck for each reading.