My quick thougths would be to keep it lower during the week for non use, but only a few degrees lower. Say keep it set at 98, and when you get to the house, kick it up to you desired temp and whithnin an hour it will be ready.
The only benifit I could see of keeping it up to say, 103 would be If there was a power outage, those few extra degrees might buy you some more time., how much time, I really don't know. But I don't think you'd want to keep the tub running all week at max temp.
However, The downside of turning it down very low, say low 90's is; more time to heat up.and less residual heat reatined in the unit in case of power loss,.
However, I think your "real' question is; Does it cost more to heat up 400 gallons of water 10 degrees once a week for a few days and then drop it down to 90, or does it cost more to keep it consitent at a higher temp say 100?? I think that is more dependant on how well the tub is insulated, the cover thicknees and the outside ambient temp.
I'd recomend a weekly standby winter temp of around 98 for peroids when it wouldn't be used. This will alow for fairly rapid heat up, decent residual temp in case of power loss, and not having the heater running all the time in case of extreme cold and rapid heat loss.
Any disagree, or am I ignorant/unaware of something?