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 $109.40   11.201%   $121.6539   $121.65   $121.65   $121.70  
  Purchase #2 $45.83   13.279%   $51.9158   $51.92   $51.90   $51.90  
  Purchase #3 $7.27   7.503%   $7.8155   $7.82   $7.80   $7.80  
  Purchase #4 $222.81   10.547%   $246.3098   $246.31   $246.30   $246.30  
  Purchase #5 $134.93   10.584%   $149.211   $149.21   $149.20   $149.20  
  Purchase #6 $142.59   7.403%   $153.1459   $153.15   $153.15   $153.10  
  Purchase #7 $73.37   8.626%   $79.6989   $79.70   $79.70   $79.70  
  7 purchase totals = $736.20       $809.7508   $809.76   $809.70   $809.70