Reservations for 1st Class Ed-York-London?

With first class, you should be OK. Trains leaving London in the evening rush hour (~4pm to ~6 pm) can get full, and you may need to stand in second class if you don't have a reservation, but unless you are very unlucky, first class should be OK.

Remember that seat reservations are free if made up to a day ahead at train stations with your pass, so if you don't use the reservation, you haven't lost any money.

Thanks, I may just pick a good contender and make a reservation to cover my bases. I just would feel bad about not filling it if I decide to leave earlier or later. Or is that not the faux pas I would expect it to be?

Ginger, it wouldn't be a Faux pas. "no shows" for reserved seats are common. Especially when business travellers have "open" tickets and their work/meeting goes on longer than planned.
Once the trains has left the station, it is legitimate to sit in an empty reserved seat. But, if the person turns up (they may have been in the toilet or buffet car) you have to leave and find another seat.

The reservation is shown by a little card in a slot in the top of the seat. It may, for example show "reserved Newcastle to Peterborough", so if you get on in Edinburgh, you can have the seat until Durham. And, if nobody turns up at Durham, you can keep it.

Well then, it sounds like I have nothing to lose by making a reservation for each leg then. Thank you!

You might like this:>http://www.seat61.com/london-to-edinburgh-by-train.htm#journey