[Joss]

...of seeing Maxwell as default renderer every Maya start:
a. Find two files in Mayall directory: mayallcreateui.mel(line 419) and mayallstartup.mel(line 425)

b. Find this line in them(line numbers above are for RC5):

setCurrentRenderer "maxwell"; //Set the current renderer Maxwell.

c. Place two slashes in start of this line like this:

//setCurrentRenderer "maxwell"; //Set the current renderer Maxwell.

That's all.