It doesn't have to be perfect. As stated above, if you don't keep the nipple square, the cap will not seat properly. Machining the hammer face is a more permanent solution, but it is not as easy as machining the cap. Also, that machined cap will likely not fit another gun properly, so you will have to keep it matched to the gun you machined it on. If you only have one gun, not a problem. I got a dozen, more or less, and sometimes I have more than one nipple in the soapy water at a time.
Best to keep it as simple as possible and the maxim "don't fix it if it ain't broke" often applies to muzzleloaders. If you are getting consistent ignition, thank God and pass the powder.