i would just add a conditional statement to set dates to an absurd range when they are missing. i hate using greater than, less than in dates (for some reason)
ie...
create procedure sp_get_report(@datefrom date=null,@dateto date=null,@customerfrom nvarchar(40)=null,@customerto nvarchar(40)=null,@salesrepfrom nvarchar(40)=null,@salesrepto nvarchar(40)=null)
as
begin
if @datefrom is null set @datefrom = '1/1/1900'
if @dateto is null set @dateto = '1/1/2050'
etc...
select total from employee where alldate between @datefrom and @dateto
and customer between @customerfrom and @customerto
and sales between @salesrepfrom and @salesrepto
end