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 $127.55   6.660%   $136.0448   $136.04   $136.05   $136.00  
  Purchase #2 $243.33   9.163%   $265.6263   $265.63   $265.65   $265.60  
  Purchase #3 $237.29   12.703%   $267.4329   $267.43   $267.45   $267.40  
  Purchase #4 $237.93   4.376%   $248.3418   $248.34   $248.35   $248.30  
  Purchase #5 $112.37   11.829%   $125.6622   $125.66   $125.65   $125.70  
  Purchase #6 $148.73   5.182%   $156.4372   $156.44   $156.45   $156.40  
  Purchase #7 $42.78   9.393%   $46.7983   $46.80   $46.80   $46.80  
  7 purchase totals = $1,149.98       $1,246.3435   $1,246.34   $1,246.40   $1,246.20