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 $61.87   7.412%   $66.4558   $66.46   $66.45   $66.50  
  Purchase #2 $59.57   3.116%   $61.4262   $61.43   $61.45   $61.40  
  Purchase #3 $52.89   5.244%   $55.6636   $55.66   $55.65   $55.70  
  Purchase #4 $215.54   9.559%   $236.1435   $236.14   $236.15   $236.10  
  Purchase #5 $102.44   12.300%   $115.0401   $115.04   $115.05   $115.00  
  Purchase #6 $222.84   5.796%   $235.7558   $235.76   $235.75   $235.80  
  Purchase #7 $32.72   4.840%   $34.3036   $34.30   $34.30   $34.30  
  7 purchase totals = $747.87       $804.7886   $804.79   $804.80   $804.80