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 $12.82   3.637%   $13.2863   $13.29   $13.30   $13.30  
  Purchase #2 $205.41   6.628%   $219.0246   $219.02   $219.00   $219.00  
  Purchase #3 $55.26   10.103%   $60.8429   $60.84   $60.85   $60.80  
  Purchase #4 $69.72   6.245%   $74.074   $74.07   $74.05   $74.10  
  Purchase #5 $134.97   12.323%   $151.6024   $151.60   $151.60   $151.60  
  Purchase #6 $61.64   13.477%   $69.9472   $69.95   $69.95   $69.90  
  Purchase #7 $242.75   9.335%   $265.4107   $265.41   $265.40   $265.40  
  7 purchase totals = $782.57       $854.1881   $854.18   $854.15   $854.10