I've been operating under the assumption that; if a word is in the dictionary, then it can not be registered as a trademark. Therefore is it safe for me to use it as a domain name?
The existence of a word in a dictionary could be a strong argument in favour of registration. Let me explain.
The dictionary gives words a meaning. You can not register the mark for products of that description. You can register the mark for all other products. Allow me to demonstrate. Start by taking any dictionary and opening it to any page. Now select any word from the page and read its definition. I have done this and selected the word "muller" defined as being "a stone or piece of metal, wood, or glass used as a pestle".
Therefore, by definition, you can not register the word "muller" to cover "a stone or piece of metal, wood, or glass used as a pestle". However, the word "muller" could be registered to cover clothing, or ice cream or a long list of items that are not made of "a stone or piece of metal, wood, or glass used as a pestle".
Let's assume you wish to offer web site development services and use the name "muller" as your domain name. When you start marketing you web site services using this the name "muller" you are in fact creating a whole new definition for the word. The word "muller" now means web site services and thus it has a secondary meaning. Words taken from a dictionary and given a secondary meaning become strong marks as there is a truly unique new meaning to the dictionary word that did not exist prior to your use.
Now that the word "muller" has a unique new meaning, and is associated with the services you offer, it becomes your trademark. It clearly distinguishes your web site services not only from all other web sites but also from any item made of stone or a piece of metal, wood or glass.