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 $18.90   13.619%   $21.474   $21.47   $21.45   $21.50  
  Purchase #2 $132.56   4.395%   $138.386   $138.39   $138.40   $138.40  
  Purchase #3 $39.72   8.666%   $43.1621   $43.16   $43.15   $43.20  
  Purchase #4 $121.95   5.536%   $128.7012   $128.70   $128.70   $128.70  
  Purchase #5 $180.97   7.314%   $194.2061   $194.21   $194.20   $194.20  
  Purchase #6 $225.99   5.832%   $239.1697   $239.17   $239.15   $239.20  
  Purchase #7 $22.33   12.758%   $25.1789   $25.18   $25.20   $25.20  
  7 purchase totals = $742.42       $790.2780   $790.28   $790.25   $790.40