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 $111.67   11.482%   $124.4919   $124.49   $124.50   $124.50  
  Purchase #2 $14.38   5.327%   $15.146   $15.15   $15.15   $15.10  
  Purchase #3 $28.86   13.487%   $32.7523   $32.75   $32.75   $32.80  
  Purchase #4 $176.49   3.804%   $183.2037   $183.20   $183.20   $183.20  
  Purchase #5 $248.31   9.184%   $271.1148   $271.11   $271.10   $271.10  
  Purchase #6 $200.77   13.278%   $227.4282   $227.43   $227.45   $227.40  
  Purchase #7 $21.84   4.759%   $22.8794   $22.88   $22.90   $22.90  
  7 purchase totals = $802.32       $877.0163   $877.01   $877.05   $877.00