IFR general aviation pilot experienced a 100 FT assent from assigned altitude because of turbulence; reporting same to ATC; fellow pilot suggesting not to admit slight altitude variations to ATC.

Narrative

I had departed BFF in VFR conditions. The automated weather was reporting a scattered layer of clouds at around 5000 FT AGL. It was my intention to climb up above the scattered layer in VFR to a cruising altitude of 9500 FT. After I realized that would be difficult; as the clouds were tending to broken rather than scattered; I called Denver Center and requested an IFR clearance to VFR on Top. Denver Center cleared me to my destination of FBL; advised me to climb and maintain 11;000; report VFR conditions; and if not in VFR conditions to maintain 11;000 FT and contact Center. Upon reaching 11;000 FT I was just on top of the cloud layer and could not maintain VFR cloud separation 100 % of the time. I advised Center and I remained on my IFR flight plan. For most of the flight I was clear of clouds; but every few miles I would fly through a cloud top. The turbulence in the cloud tops was light but always present when I passed through a top. Shortly after Denver Center handed me off to Minneapolis Center I flew through a cloud top and quickly gained 100 FT. Immediately thereafter Minneapolis Center asked me to confirm I was at 11;000'. At the time of the call I was already correcting back to 11;000' from 11;100'. I advised Center that I had been blown up to 11;100' but was back at 11;000' by the time I responded. Center acknowledged my transmission and nothing further was said about the issue. There were no traffic conflicts that I was aware of and as far as I could tell there was no other IFR traffic near my position. After I landed I was speaking with an exceptionally experienced ATP who told me that the only mistake I made was admitting to Center that I was off altitude momentarily by 100'. He told me I should never admit to being off altitude; that transponders/radar was not always accurate; and I should always advise center that I was on altitude. I am writing this report for two reasons. First; in the off chance that my altitude deviation was reported by MPLS Center I want to be protected if possible by the ASRS system as the error was inadvertent. To prevent this in the future all I can do is keep practicing my altitude holding. But I know that every once in a while I will run into turbulence that will result in a deviation even if small. But there is a second reason I am reporting this as well. My ATP friend's reaction is not unique. I have heard the comment from many pilots and I think it is a real problem. I believe that the relationship between pilots and ATC must be based on honesty and trust. I felt I was doing an excellent job maintaining altitudes through the cloud tops. I am convinced that if the flight had been my IFR check ride I would have passed with flying colors. But yes I got off altitude momentarily. I fly a light airplane; I am still learning to fly in IFR conditions; have very little actual experience (3-4 hours); and I need to fly in the clouds to improve. But how am I to get that practice if I hesitate from flying in the clouds because a slight momentary variation in altitude in turbulence might lead to a violation. I also don't want to worry constantly about being honest with ATC. I don't know what the answer is but I would think the FAA would want to encourage pilots to be totally honest about every phase of every flight.