You can do all quests up to Cristal Spire at lvl 60, 45, and 30! No need to up more as you don't need to be hl to complete them. That's why reverse rebirthing is the best solution to get sockets at Cristal spire 5 times!
4) Rebirth at 90-30-45-60-75 (Reverse order)
The pros of this method is that you get over the whole grinding to 90 aspect first. You also experience the whole content the game has to offer and be able to make a decision if you want to become even more powerful through rebirths or not (though I very much recommend it, especially that you can do the level 30-45-60-75 rebirths all in a week or so). Combatwise this is the most challenging way to do it, but after 90 it offers the quickest ramp of power of any methods (also if you gain a few elder ranks at level 90 you will get a permanent XP boost making leveling up again even faster). Another pro of this method is take you can take the full advantage of unique rewards (see next section).
Just reread the top post on the first page (been a few years since I last looked at this thread) and realized that it hasn't been updated since the level cap was raised from 90 up to 95. It should be edited to read "also if you gain a few elder ranks at level 95."