I had always heard some people say that ''It takes more energy to heat up something cold than to keep it warm all the time'', but was rather skeptical. Thanks for confirming my assumption that it's better to only heat as needed.
As for timers, I've got the spa connected to an 1800 watt timer and have ordered a 240V 7800W digital model for the water heater. No point in paying to heat the water when I'm at work or asleep.