using WM_APP range is a bit safer, but it may happen that someother application might also be sending messages of the same ID as yours,
who knows u are using WM_APP+100
someone else is using the same WM_APP+100 and sending or posting this message as a broadcast....

so theres a very rare chance for any ambigous message processing, but to be on the safer side always register a message before sending or posting.
cheers