Like has been said, if it is for the safety of flight and you can't talk to ATC then do what you have to do, however, to deviate from an ATC clearance, you do need to get approval unless you are invoking your command authority. The command authority however won't relieve you from an investigation if one is warranted.
I have had many aircrew feel that they should be able to deviate anytime that they ask for it. They don't seem to understand that we can only approve the deviation as long as we have no traffic that will be in their way as well as we are not going to violate someone elses airspace prior to getting thier approval. Quite often too, to just go around some bumps and not real large convective activity...
regards