Should I buy a JR Pass?

Hi,

I will be going to Nagano for 3 days then back to Tokyo for sightseeing for another 2 days.

If i buy the 7 day JR Pass,
I can use it for for Nex Narita Express to Tokyo,
then switch over to the Asama Shinkansen Line to Nagano. RIght?

But I can't use it for the bus ride from the Nagano station to Shigakogen, right?

Another question is when I'm back in Tokyo, can I use it to go from maybe Shibuya to Ginza or Shijuku; as in can I use this pass to get around Tokyo city?

I'm leaving Tokyo within 7 days of the activation of the pass; I can use it to get from Tokyo to Narita Airport, right?

Was just wondering if its cheaper for me to buy separate tickets for each journey or I should just get the 7 Days JR pass?

Thanks!