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 $28.81   12.747%   $32.4824   $32.48   $32.50   $32.50  
  Purchase #2 $203.57   11.584%   $227.1515   $227.15   $227.15   $227.20  
  Purchase #3 $210.38   3.349%   $217.4256   $217.43   $217.45   $217.40  
  Purchase #4 $234.53   13.413%   $265.9875   $265.99   $266.00   $266.00  
  Purchase #5 $33.44   4.656%   $34.997   $35.00   $35.00   $35.00  
  Purchase #6 $133.52   5.190%   $140.4497   $140.45   $140.45   $140.40  
  Purchase #7 $230.83   9.271%   $252.2302   $252.23   $252.25   $252.20  
  7 purchase totals = $1,075.08       $1,170.7239   $1,170.73   $1,170.80   $1,170.70