What I was trying to say was if there was a legitimate reason, like the Solyndra folks not testifying before Congress because they face criminal charges and anything they say to Congress can be used in the criminal trial, then "plead the 5th". If you just do it to do it without a very compelling reason, you tend to look guilty.
Think back several years to the baseball steroid inquiries. Remember the panel with Barry Bonds, Sammy Sosa, Mark McGwire, and Rafael Palmiero? At that panel Bonds said "I never knowingly used steroids" and the feds tried to charge him with perjury.
But perhaps more infamous than that was McGwire's insistence that he "wasn't there to talk about the past." That seemed like a very odd thing to say, and he was vilified for it in the press and general public. Why would he do such a thing? Well, the Congressional panel asked him if he ever used steroids. He did not want to answer in the affirmative (indeed, he spent the better part of a decade trying to escape culpability for his cheating). He knew that if he said 'no' he would be perjuring himself. He (and his attorneys) also knew that if he openly claimed 5th Amendment protection before Congress, with no pending investigation or litigation, it would be the same as admitting guilt. So they crafted his non-answer answer which technically was not an admission, technically was not a denial, and technically was not a refusal to answer. It was their attempt at controlling the negative PR they knew would occur with an invocation of the 5th. Unfortunately for them the Congressional panel, the media, and the public saw right through their facade and branded him a cheater and guilty.
Oh, and 21 months ago McGwire confirmed everyone's belief and admitted to being a cheater.