Is not testifying on your own behalf usually a sign of guilt in many cases?

First of all, I do not understand why a person would not want to get on the stand to try to defend themselves. Especially if they are suppose to be innocent. Now I know in this day and age more than ever before, lawyers are instructing their accused clients not to testify on their own behalfs. Case in point right now, the Casey Anthony trial. Her lawyers don't want her to talk and testify. However, I don't think that it would make very much difference in her case, because she has dug a big enough hole for herself with all her lies up to this point as it is and she looks guilty as sin. It would seem that everyone would try to plead their case and testify on their own behalf, high profile celebrities and all.