Hello 7window,
The best answer is really it depends. (For reference, I worked my way through undergraduate school working in a bicycle shop both as sales and a mechnic.)
It does not take a professional bicycle mechanic to put together a huffy bmx bike. Anyone who is mechanically inclined can to a good job at that level, as long as the wheels are ok out of the box.
However if you are building a three thousand dollar bicycle to ride across the country with from scratch, you want to pay to have a professional assemble it for you.
So to give you a good answer I need more information, both about the bicycle and you. Are you putting together a full suspension Santa Cruz mountain bike from the frame up, or a Little princess single speed for your daughter? Are you the kind of guy who rebuilds the carburetors on his old cozzie when hes bored, or are you afraid to change the oil in your car because you might strip out the oil plug?
A cheap single speed department store bike can be but together with an ok set of craftsmen tools, but there is no way anyone is touching my Waterford bicycle with a crescent wrench. To do a good job with a nice bicycle you will need several hundred dollars of specialized tools.
Where do you and your bike fall in this spectrum?