I've spotted numerous times at SDF. As Jehan stated above, there is a very good location at republic and the cellphone lot. The security has become more photographer friendly in the past few years. If they do come, they will most likely run your driver's licence through a database to see if you are actually up to no good and then let you carry on with what you're doing. The 2 weeks leading up to Christmas is probably the best time to spot the heavies at SDF as UPS is operating at peak capacity. They even contract out to other carriers such as Atlas and Centurion. The 2 days leading up to the Kentucky Derby (1st Saturday in May) are also some of the busiest. If you want to catch the huge amount of bizjets during that time, spot 35R/17L. UPS primarily uses 35L/17R. The locations that I use are below.
SDF cellphone lot:
view of the approach end for 17L Great for afternoon spotting.
SDF parking garage top deck looking west:
View of the approach end of 17R. Good for morning spotting. The parking garage is $2 per hour
Republic airways hangar:
https://goo.gl/maps/86L9yWmMJYvview of 35L/17R, also get taxi shots of aircraft leaving the worldport. Great for afternoon spotting. My personal favorite spot. As Jehan said, either bring a ladder or stand on top of your car.
UPS lot south of grade ln:
https://goo.gl/maps/ua6TD38zG8oAfternoon spotting of aircraft approaching 35R.
UPS visitor lot:
https://goo.gl/maps/RLcZcGjmrSxmorning spotting of 35L arrivals. keep a low profile because UPS is not the most photographer friendly. I normally stay in my car and shoot out the open window when i spot here.
I would avoid the FBO on the east side of the airport. I hope this helps.