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 $28.80   7.226%   $30.8811   $30.88   $30.90   $30.90  
  Purchase #2 $109.26   3.325%   $112.8929   $112.89   $112.90   $112.90  
  Purchase #3 $192.31   6.526%   $204.8602   $204.86   $204.85   $204.90  
  Purchase #4 $223.64   11.328%   $248.9739   $248.97   $248.95   $249.00  
  Purchase #5 $223.67   10.607%   $247.3947   $247.39   $247.40   $247.40  
  Purchase #6 $157.81   12.475%   $177.4968   $177.50   $177.50   $177.50  
  Purchase #7 $87.83   11.864%   $98.2502   $98.25   $98.25   $98.30  
  7 purchase totals = $1,023.32       $1,120.7498   $1,120.74   $1,120.75   $1,120.90