Should I powerwash and waterseal a pressure treated deck?
It's always good to inspect your deck every year after winter. Check for any loosening fasteners, any signs of rot, check for mold, and inspect places where wood meets wood.

I would advise against an annual power-washing. It is extremely harsh on the wood, even if done by a professional. It requires a lot of work to restore the surface, as the pressure washer damages the surfaces. I would look it at as something you do to restore an old deck.

It's usually a good idea to give the deck a gentle yet thorough annual cleaning to remove any built up dirt. If you see mold forming, it doesn't hurt to use some bleach. I'd try to do it when you have stretch of good weather so it can thoroughly dry, but avoid doing so when it's so hot that the water dries before you have a chance to rinse the cleaning solution off. 

Trimming back any bushes and growth near the deck before this step is also good, as it ensures the deck can dry properly.

When it rains, see if the water is beading and running off properly. If it's not, then you should consider re-sealing the deck. It's not uncommon to hear suggestions of an annual application of water sealant. I don't think it's necessary unless it's a particularly expensive deck, or your region has a climate that's harsher on the deck.

For the late fall, it's a bit late in the season and you may not get the right conditions. I'd still do a complete visual inspection. You can do spot treatments to hold you over until spring. Clean any mold spots. Clean any areas of rot, allow to dry, and then spot treat with sealant. If water is pooling on any boards, consider spot treating there.

(Also, pressure-washing and resurfacing are a lot of work, and there is a lot of room for error. Make sure you're careful to do it right.)