Hi again,
I guess looking at the numbers is the best way to decide this.
Narita Airport - Tokyo ¥ 2,820
Tokyo - Fukushima ¥ 8,750
Fukushima - Tokyo ¥ 8,750
Tokyo - Nikko ¥ 5,380
Nikko - Tokyo ¥ 5,380
Tokyo - Yokohama ¥ 470
Yokohama - Tokyo ¥ 470
For a total of ¥ 32,020 or ¥ 2,910 in savings, as the 7 day JR Pass is ¥ 29,110. Additionally the JR Pass is good for local travel on JR lines in Tokyo and on the Monorail to Haneda Airport.
The above travel prices are quoted for Shinkansen travel and a JR Pass will help you make savings if you wish to travel by Shinkansen. At the same time I must point out that there are local train options available for Nikko that cost about half the price, and it would be slightly more economic to go that way without the JR Pass.
That said, a JR Pass will be the most convenient option as its your one ticket to all your travel and you can take any train you'd like. May have been easier if I did this at the start :)
Hope this helps,
Daniel