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 $112.47   6.785%   $120.1011   $120.10   $120.10   $120.10  
  Purchase #2 $73.34   9.265%   $80.135   $80.14   $80.15   $80.10  
  Purchase #3 $188.60   12.475%   $212.1279   $212.13   $212.15   $212.10  
  Purchase #4 $44.99   8.122%   $48.6441   $48.64   $48.65   $48.60  
  Purchase #5 $106.90   4.227%   $111.4187   $111.42   $111.40   $111.40  
  Purchase #6 $9.43   5.368%   $9.9362   $9.94   $9.95   $9.90  
  Purchase #7 $222.45   7.491%   $239.1137   $239.11   $239.10   $239.10  
  7 purchase totals = $758.18       $821.4767   $821.48   $821.50   $821.30