Billet basically means any kind of semi-finished cast, forged, or bar-stock that is used to make a product as opposed to reground cams.
All of our cams are Billet in that we start with forgings, barstock and Chill-cast ductile iron blanks. On all of our cams, we suggest using 24v or Cr (1.3", 1.5") tappets. We have had great succes using forged steel, chill-cast ductile iron, and bar-stock based cams in all engines without bushings. That being said, as cams grow larger in duration and lift, springs pressure rises and with longer duration, of each lobe, pressure on the journals rises exponentially. That is why I suggest doing bushings based on application instead of cam material. With more valves open at the same time, opened higher, with more spring pressure, bushings are a nice insurance.
On smaller cams like the 188-220, most people do not need bushings, but they do provide additional insurance against lubrication issues. Lubrication issues really should be the main discussion, cam material second. The first 10 minutes of the cams life will dictate how it works and for how long.
The main reason to go to a steel cam is for insurance against high torsional forces and harmonics in p-pumped applications. The trade off for the lower chance of cam breakage is slightly higher chance of galling a journal in the first five minutes of run time. On common rail engines, I would stick to cast. On p-pumped applications, I still prefer cast cams in most instances. When you start spinning over 4,500 rpm and getting in and out of the throttle quickly with a mechanical pumped engine pushing over 600cc of fuel, there is a benefit in steel cams. If you are in this club that can benefit from a steel cam, do bushings as well.
On my engine last year, I was running a very healthy cam and 13mm pump and spun it 6,000 rpm with my cast cam and non-bushed block. This year I have a steel cam with no bushings, but I pay special attention to my lubrication.