Does FAR part 121 require you to have an alternate even if the weather past/current/forecast is high VFR?

As long as the weather at  your intended destination is "BELOW" minimums - (the ceiling will be at least 2000 feet, and the visibility will be at least 3sm), one hour before and one hour after the estimated arrival then an Alternate is required.

 

So in essence:
No alternate airport is required if for at least 1 hour before and 1 hour after the estimated time of arrival at the destination airport the appropriate weather reports or forecasts, or any combination of them, indicate

(1) The ceiling will be at least 2,000 feet above the airport elevation; and

(2) Visibility will be at least 3 miles.

Thanks Simeon, for shorter hops I've always wondered as the fuel to the alternate may be more than the fuel to get to your destination.

One way to work arounds that I use in the real world for planning especially at night when there are not too many options for alternates, is simply to figure the fuel required to an alternate that I would use (preferably where the weather may not be an issue), and add that is extra fuel.  So that in the unfortunate event that you get to your destination and are unable to land for whatever reason you have an option and are not forced to declare an emergency.

One way to work arounds that I use in the real world for planning especially at night when there are not too many options for alternates, is simply to figure the fuel required to an alternate that I would use (preferably where the weather may not be an issue), and add that is extra fuel.  So that in the unfortunate event that you get to your destination and are unable to land for whatever reason you have an option and are not forced to declare an emergency.

 

Always a good rule of thumb, if there isn't a viable alternate close I have also used my departure airport and have planned for trip+30 minute hold+return. This is sometimes the best option for smaller aircraft which are weight limited, turboprops and smaller regional jets.

That plus the fact, from an operational perspective going back to the departure airport especially if it is a hub is better for the passengers, as it is usually easier to provide accommodations.