tl;dr It depends on play style, slowly getting through the content or rushing into the breach getting level 32 on day one of Dark Below release (or 34 on HoW). Or whatever appeal the class has to you, if the ability to throw consecutive nova bombs turns you on, then by all means grind to your heart's content. Its your choice, but its also dependent on your style of gameplay. If you're that guy whos way into destiny and you want to get good stuff for a titan asap then by all means make 3 titans it allows you to farm stuff on one character then transfer the loot to the other 2. (i.e do crotas end on whichever one is best suited, distribute the armor so theyre all moderately equipped for crotas end then farm the raid on all three which will ultimately get all three characters to 32 quickly, same goes for HoW gear) I personally made one of each class so as to fully explore the game and pace myself so I dont get all the way through with it then get bored with it. I absolutely love being able to go invisible and stalk across the battlefield dodging that level 35 Hallowed Knight or summoning my Golden Gun and massacring hordes. Or just being that guy you can't live without because he's running Helm of Saint-14 with WoL (I hate striker ._.). Or saving everyone with my self res or just throwing consecutive nova bombs (Obsidian Mind).