wrote:

The weather on the Pacific Coast from Northern California to British
Columbia tends to be warm and dryish in the summer but cool and damp
in the winter...

Maybe not. NREL says Seattle has these average temps and humidity ratios:

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

40.1 43.5 45.6 49.2 55.1 60.9 65.2 65.5 60.6 52.8 45.3 50.5 F

..0042 .0045 .0046 .0051 .0061 .0072 .0082 .0085 .0078 .0065 .0051 .0044

The humidity ratio w is the number of pounds of water vapor per pound
of dry air. It does not depend on the air temperature, and it doesn't
change much in 24 hours. The relative humidity is the number of pounds
of water vapor per pound of air divided by the maximum number of pounds
of water vapor the air can hold (at 100% RH) at a certain temperature.

...So while for much of the continent, the season for dehumidifying
is the summer, on the left coast it is the winter.

That could be true in a Canadian airtight house with humidity sources,
unlike air-leaky US houses with energy-wasting winter humidifiers...

Dehumidifiers produce heat, therefore their energy efficiency is
important in continental applications. However, on the left coast,
they are used mostly in the winter, so the heat they produce is
mostly a slightly more expensive form of something you're going to
do anyway: heat the air in the house.

It's cheaper than electric resistance heat, with a COP of about 1.6.
You can measure this with a Kill-a-Watt meter and a measuring cup.

On the other hand, we may allow our rooms to cool below 65 degrees...

Good.

Any suggestions?

Turn on a small exhaust fan with a humidistat when the indoor RH rises to
60%. In January, w = 0.0042 makes Pa = 29.921/(1+0.62198/w) = 0.201 "Hg.
Indoor air at 60% RH and absolute temp T (R) has Pi = 0.6e^(17.863-9621/T),
approximately, and Pa = Pi makes T = 507.5 R or 507.5-460 = 47.5 F, so
you can dehumidify the house with an exhaust fan as long as the indoor
temp is at least 47.5 F. If you want to save more energy, take advantage
of weather fluctuations and hygroscopic house materials and do this less
often, only when the outdoor air is warmer and drier than average (during
the day) in wintertime and cooler and drier (at night) in summertime.

Finally, wouldn't a dehumidifier be more efficient if it took the
warm moist air in at ceiling level, then expelled warm dry air at
floor level?

Maybe not. Diffusion and convection make the water vapor pressure and the
humidity ratio of the air near the ceiling and floor about the same, even
though humid air rises.

Nick