It's never too late to start on getting your relic weapon upgraded, but realize that there's a ton of work involved and judging by what they've said when the Atma grind came out, I don't think there's going to be any point where you can skip any of the grinds. It could also be an excuse for them to suddenly include new content in later patches and make it the "strongest" weapon for that class.

Personally, I'm only going to do it for my Bard and my Ninja when it comes out because those will probably be the two classes that I love playing the most. I'm also on the fence for Black Mage, but I have a love/hate relationship with the look of that staff (Nexus looks amazing on it though).