Question about SPI_prepare

I have an application where I am building a plan with SPI_plan and
then this plan is called multiple times. There is one free parameter
($1) to the plan. The issue is with the order of the values returned.
If $1 is identical during successive calls to SPI_execute_plan, is
there any guarantee that the rows will be returned in the same order?
I know that repeated queries can return rows in different order, but
am specifically wondering about a prepared plan. (I can imagine the
plan somehow snap-shots the table when prepared such that rows always
come back in the same order, but I'm guessing that is not the case.)
If not, I simply have to add an order by clause which will incur some
overhead.

THK

-- 
Timothy H. Keitt
http://www.keittlab.org/