Just bought into the game and I want to sort and sleeve the cards with deck building involved. However I want to read the fiction as lore is a huge aspect of L5R and one of the reasons I decided to pick up the game. So my question is simply can I open and sort the cards before catching up on the fiction or are there major spoilers in card form?

Not really.

Very few cards have flavour text. Some are a bit spoilery by their very nature, but if you're that worried about it the core set is safe and the fiction is organized by the cycles so when buying the expnsions you can read the appropriate fictions before.

As well the first two cycles included a story pamphlet with each pack that had a "sequel" released the next week online. They stopped doing that with the third cycle when they moved all fiction to online only.