for me...rollersetting first allows me to use less manipulation and pulling of my hair under a more gentle and consistent heat setting when drying. It also gives my hair more bounce and softer, springier, shinier curls/hair than blowdrying does. rollersetting takes me 20 min to set and 50m to dry and blow drying takes me 30m. So, I add 40 min for softer, shinier hair. It's worth it to me and I don't consider it a waste.
If I want it straight I flat iron the whole hair with a real quick pass on the ends to smooth out curls. Doesn't take much heat or effort at all since the hair is already smooth via the curl. (My avi is 22 week post relaxer 4a-ish hair where I opted to flat iron from root to tip after a rollerset.)
Most times I only flat iron roots (I'm 22 weeks post relaxer) and leave the curls in the ends so I have lots of body, movement and full curls. It just depends on how you like your hair and what works best for it. Dif people have dif techniques that work best for their hair