I don't charge any tips for crafting when people ask me, still most of them tip me small amounts voluntarily, I've had some offer me mats and I'll gladly take those too.. But which of the two is preferred, I'd say gil since it's easier to hold on to, no organizing necessary.

Shard/crystals/clusters are nice too but you may be offering something to a crafter that's completely unnecessary where as gil always has value.. I so rarely use lightning element (WVR) that it's mostly just sitting on full stack, same with fire. If I have the inventory space I'll gladly take it, but given a choice, I'd much prefer a wind or ice (depends on the person) then given another choice, gil over everything else