First, know the means with which you are going to be able to communicate with your spotters. Since you don't utilize Ham, this could be a challenge.
Each situation is different based on the your specific coverage area, severity, direction, and speed of the event, as well as how many spotters that you have available. You must give your spotters ample time to get into position if they are going to be mobile. Not all spotters can be mobile and must do their spotting from a fixed location.
Discuss the subject with your spotters and get their opinion. Know where their starting point is. Every spotter has their favorite locations to view from, so I would recommend finding out where those locations are and use the info as a reference point to position them. Many spotters will self-activate and it will be handy to know which one's, but they may still rely on you to help get them into the best position as the event develops.
Don't hesitate to contact the NWS that covers your county to get some advice, either. They should be happy to hear from you and offer suggestions.
Hope this helps.