According to Art :
I wouldn't fluctuate it at all without a special programmed themostat
designed to prevent backup heat going on when exiting setback cycle.

You ask an interesting question about heatpumps. I would presume that the
controls for the heatpumps are designed to be smart enuf that you don't have
to worry about bypassing the heatpump. The bypass is for if the heatpump is
broken.

Right. A heat pump control system is designed to switch over to backup heat
when it makes economic sense to do so [+].

It's a balance between HP/outside temperature efficiency and
cost per BTUs in the backup system.

If it was installed/adjusted correctly and is operating properly,
don't second guess the control system, it'll do a much more effective
job at minimizing cost than you can erratically flipping a switch.
Even if the propane cost has shifted around a bit.

In other words, leave it in auto, unless the control system (or HP)
is outright failing and can't keep your house warm.

[+] The other time a HP switches to backup heat is if the inside
temperature is "too far" away from the thermostat set-point. The
threshold for "too far" is usually something on the order of 3-5
degrees. The control circuitry figures that the HP is unable to keep
up if the temperature difference is too high, and switches on emergency
heat to catch up. This makes particular sense when you realize that
a HP can't produce as many BTUs per hour as a straight gas furnace.

So, if you open all the doors and windows during a blizzard, the control
system automatically notices the gross temperature difference, and switches
on backup to try and get the temperature back in the comfort range as soon
as possible.

The difficulty arises with setback thermostats. They cause large abrupt
changes in the T-stat setting. "Oops, we can't keep up, switch on the backup!".
You need to use special programmable thermostats that either bring up the temperature
slowly (within the HP's ability to keep up - some of these T-stats have programmable
ramp-up rates I think), or inhibit backup heat during a catchup interval.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.