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 $17.96   10.750%   $19.8907   $19.89   $19.90   $19.90  
  Purchase #2 $215.32   7.481%   $231.4281   $231.43   $231.45   $231.40  
  Purchase #3 $106.72   12.859%   $120.4431   $120.44   $120.45   $120.40  
  Purchase #4 $135.43   5.736%   $143.1983   $143.20   $143.20   $143.20  
  Purchase #5 $41.78   7.447%   $44.8914   $44.89   $44.90   $44.90  
  Purchase #6 $154.78   13.177%   $175.1754   $175.18   $175.20   $175.20  
  Purchase #7 $174.77   6.381%   $185.9221   $185.92   $185.90   $185.90  
  7 purchase totals = $846.76       $920.9491   $920.95   $921.00   $920.90