To expand on Bud's post - as he says, flair is enough if bullet will start OK - but sometimes if loading lead bullets you have to flair a bit more to avoid shaving lead.
It's a compromize too - each time you flair you work harden the brass and so speed up the time to failure (splitting). You have to experiment and set up to suit the set of components you are working on.
Crimp - another thing to try and get right - ''enough but not too much'' - which means squat really! Thing is - roll crimp for instance on hot loads (357/44 etc) may be needed to be fierce to hold bullet when under recoil inertial forces. However - this is like the reverse of flair - it work hardens the brass, so - try to keep to minum that does the job.
Taper crimp on headspacing rounds like 9mm and .45 etc - needs to be firm and sometimes neck tension after resizing does most of the hold - but again - not to be overdone.
I think Lyman or RCBS do a factory crimp die too but the Lee is maybe best known. Go to
http://www.leeprecision.com/ where you can read a bit about it. I use mine most for hot loads and rifle - the plinking std loads just get crimp at seating stage.