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 $12.92   7.491%   $13.8878   $13.89   $13.90   $13.90  
  Purchase #2 $5.81   5.487%   $6.1288   $6.13   $6.15   $6.10  
  Purchase #3 $27.40   4.379%   $28.5998   $28.60   $28.60   $28.60  
  Purchase #4 $163.60   3.613%   $169.5109   $169.51   $169.50   $169.50  
  Purchase #5 $79.90   8.716%   $86.8641   $86.86   $86.85   $86.90  
  Purchase #6 $145.79   12.308%   $163.7338   $163.73   $163.75   $163.70  
  Purchase #7 $107.61   10.698%   $119.1221   $119.12   $119.10   $119.10  
  7 purchase totals = $543.03       $587.8473   $587.84   $587.85   $587.80