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 $9.93   4.234%   $10.3504   $10.35   $10.35   $10.40  
  Purchase #2 $12.56   12.376%   $14.1144   $14.11   $14.10   $14.10  
  Purchase #3 $15.49   12.305%   $17.396   $17.40   $17.40   $17.40  
  Purchase #4 $157.89   5.724%   $166.9276   $166.93   $166.95   $166.90  
  Purchase #5 $163.34   8.102%   $176.5738   $176.57   $176.55   $176.60  
  Purchase #6 $15.44   13.773%   $17.5666   $17.57   $17.55   $17.60  
  Purchase #7 $210.27   12.496%   $236.5453   $236.55   $236.55   $236.50  
  7 purchase totals = $584.92       $639.4741   $639.48   $639.45   $639.50