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 $220.98   11.562%   $246.5297   $246.53   $246.55   $246.50  
  Purchase #2 $102.76   11.453%   $114.5291   $114.53   $114.55   $114.50  
  Purchase #3 $117.45   6.223%   $124.7589   $124.76   $124.75   $124.80  
  Purchase #4 $123.99   3.692%   $128.5677   $128.57   $128.55   $128.60  
  Purchase #5 $154.86   12.617%   $174.3987   $174.40   $174.40   $174.40  
  Purchase #6 $44.31   13.693%   $50.3774   $50.38   $50.40   $50.40  
  Purchase #7 $123.43   4.761%   $129.3065   $129.31   $129.30   $129.30  
  7 purchase totals = $887.78       $968.4680   $968.48   $968.50   $968.50