Filter and stepup rings question

After a dissapointing weekend on Exmoor trying to get dramatic pictures of waterfalls I'm thinking of buying a Hoya NDx400 filter.

I'm using a Canon EOS 50D and have accumulated these lenses:

Sigma 70-300, 58mm filter
Canon 50, 50mm filter
Canon 17-85, 67mm filter
Tamron 18-270, 62mm filter

I would like to get a Tamron or Sigma Wide angle zoom with probably a 77mm filter from Santa as well.

My question is - would it be sensible to buy a 77mm filter (£80 on Amazon seems like a good price) and a set of step-up adaptor rings so I can use the filter on all my lenses - or would this be stupid?

Just to point out the obvious, but if you use a step-up ring you can't use the hood any more. I guess it isn't as important on Exmoor as it is down here.

I've heard of tripod shooters just buying the square Lee filters and holding them up in front of the lens - no need for them to be actually attached to the lens.