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 $12.29   8.772%   $13.3681   $13.37   $13.35   $13.40  
  Purchase #2 $33.80   12.116%   $37.8952   $37.90   $37.90   $37.90  
  Purchase #3 $145.63   8.265%   $157.6663   $157.67   $157.65   $157.70  
  Purchase #4 $170.51   5.558%   $179.9869   $179.99   $180.00   $180.00  
  Purchase #5 $11.88   6.564%   $12.6598   $12.66   $12.65   $12.70  
  Purchase #6 $225.71   7.357%   $242.3155   $242.32   $242.30   $242.30  
  Purchase #7 $207.40   8.613%   $225.2634   $225.26   $225.25   $225.30  
  7 purchase totals = $807.22       $869.1552   $869.17   $869.10   $869.30