with proper neck tension crimping isnt always necessary - either roll or taper. i loaded for my ar15 for about 4 years before i bought a crimp die (a Lee FCD taper crimp). While my dies are capable of applying a roll crimp, i found that by setting them that way you can sometimes inadvertently set the die incorrectly and crimp before you get fully seated. this can cause crushed shoulders, etc. and pulling a heavily roll crimped bullet is a SOB, so i just set my dies for no crimp seating. when i decided i was going to start crimping i went and got a separate die for it to make sure it was being done in the easiest fashion, and as possibly error free as i could make it.
if you want to test if you're getting any setback, load up a dummy round. just brass & bullet, but otherwise treat it as if you were loading a full cartridge. Measure it, put it in the mag and chamber it under full action from BHO either by letting the charging handle fly, or hitting the bolt release. do this 5-10 times. feel free to measure several time throughout the testing.
if your bullet is getting set back in the case, especially if its only after 2 or 3 chamberings, you should probably consider crimping. if theres no setback at all after 10 chamberings, your neck tension is sufficent and you likely have no need to crimp at all.
for me using the taper crimp die allows me to ensure that my neck tension is consistent. when i'm making hunting loads that may see 5+ chamberings during hunting season, this starts to become important. personally after i chamber any round more than 5-6 times, even if its crimped... it goes to the bottom of the mag and the next one in line takes over the job.