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 $20.25   4.530%   $21.1673   $21.17   $21.15   $21.20  
  Purchase #2 $15.78   12.215%   $17.7075   $17.71   $17.70   $17.70  
  Purchase #3 $219.94   6.560%   $234.3681   $234.37   $234.35   $234.40  
  Purchase #4 $85.73   13.353%   $97.1775   $97.18   $97.20   $97.20  
  Purchase #5 $189.37   5.687%   $200.1395   $200.14   $200.15   $200.10  
  Purchase #6 $158.51   3.125%   $163.4634   $163.46   $163.45   $163.50  
  Purchase #7 $230.31   4.565%   $240.8237   $240.82   $240.80   $240.80  
  7 purchase totals = $919.89       $974.8470   $974.85   $974.80   $974.90