All warlock or multiclass sorcerer?

[Dalebert]

3 level dip in sorcerer is a trivial cost in terms of your high level warlock features. I've always said I would never go past 17 in warlock because it's crap. May as well dip something at that point. Obviously there is a cost if you take it early as it delays your warlock advancement.

5 level dip is a sweet spot since you will cap out at 5th level warlock slots and one slot replenishes all your points. That's 10 sorcery points you can use each short rest to cook up lower level slots with (up to 3rd). How useful that is depends on how stingy your DM(s) are with short rests. Of course you give up your once per day 9th level spell and an invocation. But you get two 3rd level slots and 3rd level sorcerer spells.