reserve seats to Nikko in advance?

Aloha,
We are coming to Tokyo in late May and want to go to Nikko by train. Will buy the World Heritage Pass in advance, but wonder if I should go ahead and reserve our seats or if we can get seats 3-4 days ahead once we arrive in Tokyo. Thanks.

You can get to Nikko by JR to Utsonomiya on the shinkansen and then a local which all takes about 2 hours. From Tokyo some shinkansen require a reservation whist others do not.
You can also go by Tobu Railway from Asakusa which also takes about 2 hours but has some through trains, Some trains require reservations, others do not.
Tobu is substantially less expensive unless you are using a JR pass in which case JR wins hands down.
Reservations could most probably be made just in time unless you were travelling at a peak time such as Golden Week.

Will buy seats when we get there and see what the schedule is like. thanks for your info.