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 $18.40   12.594%   $20.7173   $20.72   $20.70   $20.70  
  Purchase #2 $110.81   5.578%   $116.991   $116.99   $117.00   $117.00  
  Purchase #3 $13.89   4.430%   $14.5053   $14.51   $14.50   $14.50  
  Purchase #4 $191.70   13.658%   $217.8824   $217.88   $217.90   $217.90  
  Purchase #5 $145.25   13.186%   $164.4027   $164.40   $164.40   $164.40  
  Purchase #6 $129.89   12.158%   $145.682   $145.68   $145.70   $145.70  
  Purchase #7 $143.30   7.444%   $153.9673   $153.97   $153.95   $154.00  
  7 purchase totals = $753.24       $834.1480   $834.15   $834.15   $834.20