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 $27.33   3.497%   $28.2857   $28.29   $28.30   $28.30  
  Purchase #2 $27.33   11.512%   $30.4762   $30.48   $30.50   $30.50  
  Purchase #3 $227.86   7.161%   $244.1771   $244.18   $244.20   $244.20  
  Purchase #4 $159.56   4.490%   $166.7242   $166.72   $166.70   $166.70  
  Purchase #5 $51.95   11.222%   $57.7798   $57.78   $57.80   $57.80  
  Purchase #6 $36.70   12.177%   $41.169   $41.17   $41.15   $41.20  
  Purchase #7 $69.63   5.358%   $73.3608   $73.36   $73.35   $73.40  
  7 purchase totals = $600.36       $641.9728   $641.98   $642.00   $642.10