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 $19.86   5.517%   $20.9557   $20.96   $20.95   $21.00  
  Purchase #2 $181.79   9.725%   $199.4691   $199.47   $199.45   $199.50  
  Purchase #3 $206.89   4.201%   $215.5814   $215.58   $215.60   $215.60  
  Purchase #4 $113.42   10.517%   $125.3484   $125.35   $125.35   $125.30  
  Purchase #5 $11.58   3.638%   $12.0013   $12.00   $12.00   $12.00  
  Purchase #6 $93.93   11.841%   $105.0523   $105.05   $105.05   $105.10  
  Purchase #7 $180.40   13.640%   $205.0066   $205.01   $205.00   $205.00  
  7 purchase totals = $807.87       $883.4148   $883.42   $883.40   $883.50