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 $123.71   3.770%   $128.3739   $128.37   $128.35   $128.40  
  Purchase #2 $120.48   12.683%   $135.7605   $135.76   $135.75   $135.80  
  Purchase #3 $232.78   9.196%   $254.1864   $254.19   $254.20   $254.20  
  Purchase #4 $47.47   9.723%   $52.0855   $52.09   $52.10   $52.10  
  Purchase #5 $93.91   13.734%   $106.8076   $106.81   $106.80   $106.80  
  Purchase #6 $24.53   8.170%   $26.5341   $26.53   $26.55   $26.50  
  Purchase #7 $244.78   6.528%   $260.7592   $260.76   $260.75   $260.80  
  7 purchase totals = $887.66       $964.5072   $964.51   $964.50   $964.60