As I understand it, in Washington State, where we are, the code requires the test now, but has no standard it has to meet. Technically, it could be done just at the end, but at that point, it is too late to fix any leaks. So if you want to be able to act on the information it provides, you'd do it after you think you have the building air-sealed, then seal up any leaks you find. Then I think you do have to do it at the end. so you get to pay for it twice! May depend on where you live, though. We haven't gotten to that point yet.

I always do my blower door after sheetrock, when
a/c condensing unit is installed I have to verify
efficiency of installed a/c unit.

of course I also do an open wall, intermediate
inspection, once house is blacked in, ductwork is
installed with furnace, windows, doors and insulation
in place. at this time I do inspection & verification
of all above, a walk around of outside & inside of
house to point out leakage areas to be addressed.

this inspection isn't manditory for my provider's
requirements but I've found that without this inspection..
the houses aren't sealed as well as with the inspection.
also..at this time there are still tradespeople on
site to do the work.
it is my personal choice to do this additional inspection.
( I think it should be manditory...but it isn't...yet)
after 15 years of doing this work..I can catch the
bulk of the leakage at this point. it will
differ depending on experience of rater/auditor/
person doing the testing.

I read of others doing testing prior to sheetrock,
but unless ceilings are solid sheeted, I don't get
how you can blower door test a house with living
space open to attic...and attic vented.

at any rate the price quoted should include any
inspections, recommendations & testing.
if the house at any point when tested had excessive
leakage...then an additional test is needed to verify
that leakage was addressed. otherwise it is just
the opinion of someone that the house achieved
its non verified degree of tightness...a wag.

in the big scheme of things..paying for a blower
door test is a minimal charge compared to other
items in the house. and as a tighter house is
easier to heat/cool...the cost is quickly recouped.

best of luck.

@lori_inthenw It is part of code starting in 2013 here in Ohio although code does not specify (that I know of) when to do it.
@energy_rater Thanks for that info. I've gotten more info from the builder today since the original post. There is going to be a preliminary inspection after insulation and before sheetrock like you suggested. The test is done after the everything is finished.

If there is only 1 test, it is done at the finish. If we do a 2nd test, then it is done after ceiling sheetrock is installed and the attic is sealed but prior to wall insulation and sheetrock. Also all RI of mechanical, electrical and plumbing is complete as these are leaky areas. IMO this the best time to do it as it finds the leaks and they are easily fixed. Typically if you get a good reading at this point, it will only be better with the final. Around here, a blower door costs about $300, so not a big deal typically if the owner is really concious of the importance of air sealing, in which I do my best to educate.
But like ER stated, its pretty easy to do a visual inspection of open framing and find the possible problem areas from doing it enough times.

Here's some reading about it.

http://www.bing.com/search?q=blower+door+test+procedure&qs=AS&form=QBLH&pq=blower+door+&sc=8-12&sp=3&sk=AS2

I've always found it to be a problem to test prior to sheetrock. once insulation is finished...they are ready to
rock.
rather than do all ceilings, around here it is ceiling first,
then walls, and on to the next room.

the only way I've ever been able to schedule the open wall inspection prior to sheetrock is because I have the homeowner call me when insulation is scheduled.
that usually gives me a day, or at the most two days
to do my inspection before the insulation is covered.

best of luck.

If the builder is new to testing, then prior to drywall is highly advised if you want to make a meaningful reduction in air infiltration. We do ours only once, whenever and consistently break the records for the tightest tests in our region. If a builder is experienced with testing and is conscious of their work then they are likely to do well at any point in the process.