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 $224.76   13.339%   $254.7407   $254.74   $254.75   $254.70  
  Purchase #2 $238.35   9.473%   $260.9289   $260.93   $260.95   $260.90  
  Purchase #3 $19.90   4.143%   $20.7245   $20.72   $20.70   $20.70  
  Purchase #4 $19.52   3.443%   $20.1921   $20.19   $20.20   $20.20  
  Purchase #5 $136.96   9.398%   $149.8315   $149.83   $149.85   $149.80  
  Purchase #6 $40.36   7.213%   $43.2712   $43.27   $43.25   $43.30  
  Purchase #7 $50.94   6.552%   $54.2776   $54.28   $54.30   $54.30  
  7 purchase totals = $730.79       $803.9665   $803.96   $804.00   $803.90