Hi Scott,
The main concern when you use flow permanent or flow timeout multiplier is the starvation of resources on the CSS.
If you need to use these commands, you need to consider how the application behaves and what are your clients needs before making a decision about how long the flow should be idle without the CSS reciclying it.
For example, if your users needs to be logged during the whole day, you might want to configure the flow timeout to 8 hours, so the flows are clean by the end of the day.
Try not to use the flow permanent, as it is likely that flows would remain idle for long periods of time.
Using the flow timeout command on the content rule needs the consideration of the amount of users per day that hit that content rule and also, at least at the beggining of the deployment, needs that you monitor the CSS' CPU and available flows.
Thanks & Regards,
Jose.