Most HVAC systems are designed for a particular outside temperature. If the outside temperature drops below that level (as it will maybe a few nights a year or a few nights a decade, depending on the design temp), the HVAC system will run continuously, unable to bring the temperature up as high as the thermostat is set.
Unless the mechanical engineer specifies otherwise, I would size the service to allow for all of the strip heaters to operate simultanously, otherwise you will be tripping breakers. Now, you *may* be able to uprate certain components based on low ambient temperature if they're in unconditioned spaces (since this will only happen when it's very very cold), but probably not. You might want to check and see what size transformer the poco has to serve this house, too, since they love to undersize those suckers and it's hard to justify a 600A service if the poco only has a 50kVA pole pig serving it.