Hey guys, it is not 100% necessary to change the tappets on 24v's and CR's unless you have obvious wear. At least clean and inspect them. And yes for what little they cost it is good insurance. Also very important, for at least break in, is the cam lube.
On 12v models due to the wider lobes we have on our core, we suggest the wider 98-02 tappets to maximize the use of all of that extra surface area, as well as promote better tappet rotation. I have seen people ignore this and they have not reported any problems, but I would change them just the same.
As far as pushrods, people say they are needed when using a performance cam, this is false. People say that they are needed with the slightly higher spring pressures we are running. This is false . They become necessary as horsepower and/or rpms increase. The higher cylinder pressures needed for high horsepower really make the exhaust pushrods bend, compress and deflect. Double, triple, or more the horsepower, on top of that, add rpm and they are definately needed. If you are under 500hp even if you have a performance cam or springs, they are not 100% necessary unless you are just preparing for future plans. If you are running a high lift cam with a small base circle they will be beneficial if you order them a bit longer. This will make sure you don't use up all of your adjuster on the rocker arm and keep your rocker geometry correct.
Sorry for the long winded answer, it is a technical question so it is not that easy to answer.