Is it always required to calculate the fuel for an IFR alternate? I was asked to plan a cross country flight from KTMB (Miami Executive Airport) to KTPA (Tampa International Airport). After doing all the planning, I realized I needed to calculate the fuel for an alternate airport.

According to 14 CFR 91.167, fuel is required from departing airport to intended destination airport, plus from intended destination airport to alternate airport, and another 45 minutes of flying at normal cruising speed.

In 1-2-3, if weather forecasts tell that 1 hour before and 1 hour after the ETA, the ceilings would be less than 2,000 feet and less than 3 statute miles visibility, I need to file an alternate airport.

But if the forecasts tell that at KTPA, the weather conditions will be better than that, would I still be required to calculate the fuel for an alternate?

If the conditions are worse than that, then how would I calculate the fuel to my alternate, which I chose to be KSRQ (Sarasota–Bradenton International Airport). Do I need to do again the SID at KTPA, then enroute and then STAR and approach at KSRQ, or just a direct flight?
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.