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 $69.86   10.655%   $77.3036   $77.30   $77.30   $77.30  
  Purchase #2 $171.95   3.228%   $177.5005   $177.50   $177.50   $177.50  
  Purchase #3 $143.25   10.192%   $157.85   $157.85   $157.85   $157.90  
  Purchase #4 $236.26   8.223%   $255.6877   $255.69   $255.70   $255.70  
  Purchase #5 $21.95   7.359%   $23.5653   $23.57   $23.55   $23.60  
  Purchase #6 $81.43   9.473%   $89.1439   $89.14   $89.15   $89.10  
  Purchase #7 $70.35   8.772%   $76.5211   $76.52   $76.50   $76.50  
  7 purchase totals = $795.05       $857.5721   $857.57   $857.55   $857.60