Technically you can, but it can set a bad precedent. If you are deadheading you are entitled to a seat in back and can always insist on that seat. If it's a long deadhead and I have to work when I arrive, I will insist. I have however voluntarily rode up front to free a pax seat WHEN ASKED NICELY. I'll only do this to allow an innocent person to get on the plane, not for the convenience of scheduling. I'll usually help out these folks...

1) An offline pilot who is not authorized the actual JS (rare now with CASS)
2) A non-rev pax who is not authorized the JS, ie non-pilot employee or somebodies Mom.
3) A revenue pax if it's the last flight to/from Podunk, MN that day. Some people disagree with this, but I am exercising my right to be nice to my fellow human beings. It's not like the company planned it this way, the situation comes up and I volunteer.

NOTE: If I see another pilot standing around the gate, and I am deadheading I will chat with him...if HE needs the jumpseat, I will stay in my cabin seat regardless.

HOWEVER...if the company (ie crew tracking, training dept, etc) EVER asks you to deadhead in the JS (they will), you MUST SAY NO. This sets a VERY bad precedent and if allowed to happen eventually the company will just assume that they can schedule ALL deadheading pilots in the JS, thereby eliminating the JS as an option for personal travel.