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 $28.91   6.834%   $30.8857   $30.89   $30.90   $30.90  
  Purchase #2 $27.27   10.231%   $30.06   $30.06   $30.05   $30.10  
  Purchase #3 $227.94   6.732%   $243.2849   $243.28   $243.30   $243.30  
  Purchase #4 $6.61   12.244%   $7.4193   $7.42   $7.40   $7.40  
  Purchase #5 $210.67   7.588%   $226.6556   $226.66   $226.65   $226.70  
  Purchase #6 $207.91   8.497%   $225.5761   $225.58   $225.60   $225.60  
  Purchase #7 $122.33   9.747%   $134.2535   $134.25   $134.25   $134.30  
  7 purchase totals = $831.64       $898.1351   $898.14   $898.15   $898.30