Is it always required to calculate the fuel for an IFR alternate?
You only need an alternate if a) the weather is below the minimums you mentioned, or b) your destination has no instrument approach. That's in both 91.167 (IFR fuel requirements) and 91.169 (IFR flight plan requirements), e.g.:


  §91.167   Fuel requirements for flight in IFR conditions.
  
  (a) No person may operate a civil aircraft in IFR conditions unless it
  carries enough fuel (considering weather reports and forecasts and
  weather conditions) to—
  
  [...]
  
  (2) Except as provided in paragraph (b) of this section, fly from that
  airport to the alternate airport; and
  
  [...]
  
  (b) Paragraph (a)(2) of this section does not apply if:
  
  [...]
  
  (2) Appropriate weather reports or weather forecasts, or a combination
  of them, indicate the following:
  
  (i) For aircraft other than helicopters. For at least 1 hour before
  and for 1 hour after the estimated time of arrival, the ceiling will
  be at least 2,000 feet above the airport elevation and the visibility
  will be at least 3 statute miles.


As for how to calculate the fuel requirement to your alternate, I would just plan a direct route (or whatever you need to avoid restricted airspace etc.) and calculate your regular cruise fuel requirement. I don't see any point in trying to anticipate in detail what would happen if you actually have to divert because you just can't know that anyway.

IFR Magazine has a nice article on planning alternates that might help you with some of the practical details.