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 $189.96   8.569%   $206.2377   $206.24   $206.25   $206.20  
  Purchase #2 $27.51   10.462%   $30.3881   $30.39   $30.40   $30.40  
  Purchase #3 $71.97   3.173%   $74.2536   $74.25   $74.25   $74.30  
  Purchase #4 $152.56   4.421%   $159.3047   $159.30   $159.30   $159.30  
  Purchase #5 $104.75   5.601%   $110.617   $110.62   $110.60   $110.60  
  Purchase #6 $233.71   13.863%   $266.1092   $266.11   $266.10   $266.10  
  Purchase #7 $145.54   7.583%   $156.5763   $156.58   $156.60   $156.60  
  7 purchase totals = $926.00       $1,003.4866   $1,003.49   $1,003.50   $1,003.50