You might want to let them roll their own but set limits.
I'd suggest no more than 4 terms and no more than 2 classes, within a single career. Depends on the type of campaign really.
Remember, the class and prior history don't need to be the same, though I suggest within each term at least one level go to the prior history career class.
Some classes go well together, given the right raison d'etre
Navy/Rogue, a naval intelligence officer.
Marine/Mercenary, a special forces commando.
Merchant/Rogue, a smuggler.
Scout/Traveller, an academic/scientifically minded scout.
One other limit you might prescribe is that the prior history class should have equal to or more levels than the secondary class.
You'll also need to talk through the process with each player. Certain feats need to reflect the characters background. ie: there should be a roleplaying reason for having them, not just cuz they add bonuses to particular things.
If this is too much trouble, or you just want to get playing quickly, maybe pre-rolled are better. You can do all the thinking for them then.