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 $135.75   12.306%   $152.4554   $152.46   $152.45   $152.50  
  Purchase #2 $46.97   10.332%   $51.8229   $51.82   $51.80   $51.80  
  Purchase #3 $235.96   13.833%   $268.6003   $268.60   $268.60   $268.60  
  Purchase #4 $226.61   3.367%   $234.24   $234.24   $234.25   $234.20  
  Purchase #5 $93.71   11.351%   $104.347   $104.35   $104.35   $104.30  
  Purchase #6 $71.95   9.779%   $78.986   $78.99   $79.00   $79.00  
  Purchase #7 $35.40   7.819%   $38.1679   $38.17   $38.15   $38.20  
  7 purchase totals = $846.35       $928.6195   $928.63   $928.60   $928.60