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 $18.91   11.545%   $21.0932   $21.09   $21.10   $21.10  
  Purchase #2 $99.66   13.529%   $113.143   $113.14   $113.15   $113.10  
  Purchase #3 $110.66   13.115%   $125.1731   $125.17   $125.15   $125.20  
  Purchase #4 $177.80   12.231%   $199.5467   $199.55   $199.55   $199.50  
  Purchase #5 $212.48   6.514%   $226.3209   $226.32   $226.30   $226.30  
  Purchase #6 $234.50   7.108%   $251.1683   $251.17   $251.15   $251.20  
  Purchase #7 $132.81   8.417%   $143.9886   $143.99   $144.00   $144.00  
  7 purchase totals = $986.82       $1,080.4338   $1,080.43   $1,080.40   $1,080.40