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 $18.83   13.644%   $21.3992   $21.40   $21.40   $21.40  
  Purchase #2 $139.49   12.743%   $157.2652   $157.27   $157.25   $157.30  
  Purchase #3 $10.72   8.488%   $11.6299   $11.63   $11.65   $11.60  
  Purchase #4 $180.41   12.499%   $202.9594   $202.96   $202.95   $203.00  
  Purchase #5 $110.50   6.852%   $118.0715   $118.07   $118.05   $118.10  
  Purchase #6 $16.25   8.402%   $17.6153   $17.62   $17.60   $17.60  
  Purchase #7 $52.89   13.701%   $60.1365   $60.14   $60.15   $60.10  
  7 purchase totals = $529.09       $589.0770   $589.09   $589.05   $589.10