Ideally you want to be as geared as possible - before taking on the next step of progression.
Once you hit 50, you can start getting

ilvl 50 Market
ilvl 55 via WP + Market.
ilvl 60 Ifrit + AK
ilvl 70 Garuda + Darklight
ilvl 80 Relic/Titan
ilvl 90 Relic +1 / Coil

You generally want to get as much as the previous tier before working to the next, this is not always the case as some fights are easier and you can get carried by other people in the group, there are fights which are more reliant on gear and some can be done totally by good execution. You will want to be almost fully ready for Titan and Coil, and be decently geared for AK since there are DPS/Healing/Tank.