Do signatures always mean the murderer is experienced? No.
I think I had a post about this a few months ago. There are two big misconceptions about "signatures." One is that since they are often used to link crimes in a series, that part of the definition is that they must occur over and over. Not true. The definition of a signature is some action that was wholly unnecessary to the commission of the crime, but that was undertaken for psychological reasons related to the underlying fantasy or psychological urges of the offender. An offender can leave signatures even in his first murder. If you are looking for an example, the one I used last time was Austin Sigg who murdered Jessica Ridgeway (signatures in this crime were hair cutting, putting her in different clothes, etc.). None of those actions were necessary to murder or assault her. They weren't necessary for him to conceal his crime. He did them because they had psychosexual meaning to him. If he hadn't been turned in, in his next crime you'd likely have seen hair cutting and re-dressing again. Or some form of those actions. Because offenders can tinker with signatures as their fantasies evolve or the exigencies of the crime change (they will likely remain similar as they revolve around the same psychosexual urges).
The second big misconception about signatures is that they are often an object left at the scene. A cross made out of twigs, a single red rose, a note left in lipstick, a heart carved in a tree or some nonsense like that. Signatures are rarely objects. Almost 100% of the time, signatures are actions taken by the offender meant to sexually demean the victim. The most common signatures are sexual posing, insertion of foreign objects, shaving of pubic hair, type of bindings.
IMO.