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 $60.68   6.542%   $64.6497   $64.65   $64.65   $64.60  
  Purchase #2 $232.87   10.776%   $257.9641   $257.96   $257.95   $258.00  
  Purchase #3 $237.97   4.521%   $248.7286   $248.73   $248.75   $248.70  
  Purchase #4 $129.78   12.107%   $145.4925   $145.49   $145.50   $145.50  
  Purchase #5 $21.38   9.110%   $23.3277   $23.33   $23.35   $23.30  
  Purchase #6 $14.77   5.471%   $15.5781   $15.58   $15.60   $15.60  
  Purchase #7 $173.37   5.833%   $183.4827   $183.48   $183.50   $183.50  
  7 purchase totals = $870.82       $939.2234   $939.22   $939.30   $939.20