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 $43.65   11.131%   $48.5087   $48.51   $48.50   $48.50  
  Purchase #2 $234.76   9.447%   $256.9378   $256.94   $256.95   $256.90  
  Purchase #3 $108.32   6.648%   $115.5211   $115.52   $115.50   $115.50  
  Purchase #4 $61.98   4.788%   $64.9476   $64.95   $64.95   $64.90  
  Purchase #5 $24.68   10.490%   $27.2689   $27.27   $27.25   $27.30  
  Purchase #6 $171.32   12.660%   $193.0091   $193.01   $193.00   $193.00  
  Purchase #7 $48.26   9.515%   $52.8519   $52.85   $52.85   $52.90  
  7 purchase totals = $692.97       $759.0451   $759.05   $759.00   $759.00