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 $177.73   4.178%   $185.1556   $185.16   $185.15   $185.20  
  Purchase #2 $205.44   5.757%   $217.2672   $217.27   $217.25   $217.30  
  Purchase #3 $199.54   13.142%   $225.7635   $225.76   $225.75   $225.80  
  Purchase #4 $141.29   11.151%   $157.0452   $157.05   $157.05   $157.00  
  Purchase #5 $25.95   10.299%   $28.6226   $28.62   $28.60   $28.60  
  Purchase #6 $95.89   12.145%   $107.5358   $107.54   $107.55   $107.50  
  Purchase #7 $63.64   9.301%   $69.5592   $69.56   $69.55   $69.60  
  7 purchase totals = $909.48       $990.9491   $990.96   $990.90   $991.00