Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $238.84   8.835%   $259.9415   $259.94   $259.95   $259.90  
  Purchase #2 $239.45   8.426%   $259.6261   $259.63   $259.65   $259.60  
  Purchase #3 $50.52   11.192%   $56.1742   $56.17   $56.15   $56.20  
  Purchase #4 $123.35   8.595%   $133.9519   $133.95   $133.95   $134.00  
  Purchase #5 $146.51   11.600%   $163.5052   $163.51   $163.50   $163.50  
  Purchase #6 $106.65   11.873%   $119.3126   $119.31   $119.30   $119.30  
  Purchase #7 $24.40   11.565%   $27.2219   $27.22   $27.20   $27.20  
  7 purchase totals = $929.72       $1,019.7334   $1,019.73   $1,019.70   $1,019.70