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 $28.48   5.685%   $30.0991   $30.10   $30.10   $30.10  
  Purchase #2 $99.55   11.334%   $110.833   $110.83   $110.85   $110.80  
  Purchase #3 $74.89   4.646%   $78.3694   $78.37   $78.35   $78.40  
  Purchase #4 $111.81   12.694%   $126.0032   $126.00   $126.00   $126.00  
  Purchase #5 $160.94   7.536%   $173.0684   $173.07   $173.05   $173.10  
  Purchase #6 $13.27   3.573%   $13.7441   $13.74   $13.75   $13.70  
  Purchase #7 $44.27   13.344%   $50.1774   $50.18   $50.20   $50.20  
  7 purchase totals = $533.21       $582.2946   $582.29   $582.30   $582.30