Install Integrity windows over Asphalt felt or over other??

[kooshball]

After reading the instructions on the Integrity windows it lists a procedure for installation over asphalt felt. Is is best practice to this is even with having to make 90deg bends in the felt to allow it into the rough opening (won't it crack)? How about getting some wide, flexible PVC and using that in place of the asphalt felt to line the rough opening before applying sealant and then the nailing fin?

Thanks