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 $37.96   11.201%   $42.2119   $42.21   $42.20   $42.20  
  Purchase #2 $117.87   5.819%   $124.7289   $124.73   $124.75   $124.70  
  Purchase #3 $147.71   11.380%   $164.5194   $164.52   $164.50   $164.50  
  Purchase #4 $58.33   3.657%   $60.4631   $60.46   $60.45   $60.50  
  Purchase #5 $15.42   9.249%   $16.8462   $16.85   $16.85   $16.80  
  Purchase #6 $203.85   10.242%   $224.7283   $224.73   $224.75   $224.70  
  Purchase #7 $245.42   9.128%   $267.8219   $267.82   $267.80   $267.80  
  7 purchase totals = $826.56       $901.3197   $901.32   $901.30   $901.20