I think when you get hired to a job that pays say for example 10k a week. You should be given one weeks pay immediately.
Why? Two reasons, one IC and one OOC.
IC: It makes sense to pay a week forward because then if they quit or are fired, they are done, no need to track down an npcs for backpay because technically they were already paid for a week they didn't do anything.
OOC: It gives the player some initial flash to play with, buy new clothes, buy a car (no idea what cars cost but making an example).
So with those two reasons both IC and OOC, I feel it makes sense to pay the employee week one at day of hire.