The order in which the parameters appear in the BIDS Report Data tab can be the difference between your report executing and it failing. Failure to pay attention to this will give you very mysterious error messages!
"The Value expression for the query parameter PARAMNAME contains and error:The expression that references the parameter PARAMNAME does not exist in the parameters collection. Letters in the names of parameters must have the correct case"
It may be visible at design time but not come into scope when the particular expression is evaluated!
You can force the order of the parameters by going into the XML code and adjusting as needed.