Seat reservations needed for English trains?

I think what trains you are thinking about taking makes the difference here. Reservations on a regional train? Probably not.
Reservations on a popular long-distance route? I would probably make them since they aren't that expensive and I tend to be picky about where I sit.

Have you checked what the cost of a reservation is on the routes you are interested in?

I would especially if it is a several hours trip. If going A to F, it may be hard to find a vacant seat for that distance. So you might have to move around as you will find that some seats are reserved fro A to C, C to F, B to D, etc.

If you are going to say Canterbury or Windsor, then you don’t need seat reservations. I don’t usually bother reserving seats and always find a seat, but I never travel peak times long distance on a Friday evening.

On lots of services there is no option to reserve seats
For example my service from Stratford upon Avon to London you can't reserve seats
On the services where you can reserve seats there is no cost to do this

We needed them York-to-Birmingham as an example, off season. Its not just tourists that use the trains. But if you're buying tickets in advance, on line, if the system offers you opportunity to make reservations, go ahead and make them.

There are certain routes where it is always advisable. For example on Crosscountry and GWR, at least to the South West especially Friday-Sunday.

I absolutely agree with Marco about Crosscountry. I don't ride GWR much. Crosscountry always seems to have more passengers than seats, and a tiny First Class area. Be sure you get one of the seats.