If you are not multi rated, and this will be fifty hours of mutiengine instruction, then signing your logbook is the logical way to go.
However, until the checkride is passed, you cannot ACT as PIC. That's important to many people.
Note: there are some wonderful discussions about all of the details of logging time on Doc's website. Some people say that there is a legitimate reason that this is the only industry where we can log an activity as PIC time, when in fact the regulations do not allow us to ACT as PIC, and vice versa.
To me, it's like Being able to "log" that I produced 1,000 widgets, when in fact I never produced a widget at all, never performed the "act" of making the widgets. It is an idea that is consistent with bureaucracies, rather than realities.
So, logging questions which would have been "simple" if they were simply a record of pilot time have become complicated by having to "fit" with other regualtions.
But I have digressed. Go to Doc's forum for now, and soon someone with more logging experience will suggest an answer that may apply specificallly to you.
http://www.propilot.com/
Go to the drop down menu on the left side, and select DOC's far forum.
If you are on a pro pilot track, be aware that there is a difference between logged PIC time and what the airlines see as
being the PIC according to FAR One, the pilot who has final responsibility for the aircraft. In the area of trainers, this would mean that the FBO is willing to rent you the airplane as the
pilot, not someone receiving instruction.