I would advise having separate profiles for SinglePlayer and Multiplayer , because of the time difference on MP. In Singleplayer, when you exit the game, the game pauses, but in Multiplayer, the time doesn't stop. Hence, leaving the job before delivering it, might result in some crazy time delay.
Also, if you have hired drivers in SinglePlayer, once you connect on Multiplayer, time synchronises with the server (usually moves time a couple of weeks forward), and you will be surprised how much money your drivers have made in this time (usually millions of dollars)
If you want to avoid this and keep it balanced and fair, keep SinglePlayer separate from Multiplayer.
For myself, i use one single profile, but i'm playing Multiplayer only , and not planning to have more than 1 truck or hired drivers.