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 $34.99   12.766%   $39.4568   $39.46   $39.45   $39.50  
  Purchase #2 $12.63   5.800%   $13.3625   $13.36   $13.35   $13.40  
  Purchase #3 $8.42   12.437%   $9.4672   $9.47   $9.45   $9.50  
  Purchase #4 $235.51   3.428%   $243.5833   $243.58   $243.60   $243.60  
  Purchase #5 $111.90   12.106%   $125.4466   $125.45   $125.45   $125.40  
  Purchase #6 $219.49   11.829%   $245.4535   $245.45   $245.45   $245.50  
  Purchase #7 $217.45   4.551%   $227.3461   $227.35   $227.35   $227.30  
  7 purchase totals = $840.39       $904.1160   $904.12   $904.10   $904.20