For my own characters and the PCs at the tables I run... I always go with seeing where the PC is at level up and then selecting/offering the mechanical options that makes the most sense for where the character is in its life.
The only exception to this are PCs that are intended to multiclass from the beginning. If a player said they wanted to make a cleric/wizard multiclass... I would build a "new" class that sets the level pattern ahead of time and they would use that. So for instance, in my last campaign a player had in mind a forge cleric / bladesinger. Rather than let him select which levels of which class he would take upon level up... I created the Forgesinger "class" whose level chart gave out the abilities of the cleric and wizard in an established pattern (3 levels wizard, 2 levels cleric, 3 levels wizard, 2 levels cleric and so on.)
The only exception to this are PCs that are intended to multiclass from the beginning. If a player said they wanted to make a cleric/wizard multiclass... I would build a "new" class that sets the level pattern ahead of time and they would use that. So for instance, in my last campaign a player had in mind a forge cleric / bladesinger. Rather than let him select which levels of which class he would take upon level up... I created the Forgesinger "class" whose level chart gave out the abilities of the cleric and wizard in an established pattern (3 levels wizard, 2 levels cleric, 3 levels wizard, 2 levels cleric and so on.)