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 $230.77   12.565%   $259.7663   $259.77   $259.75   $259.80  
  Purchase #2 $173.78   8.741%   $188.9701   $188.97   $188.95   $189.00  
  Purchase #3 $71.45   3.422%   $73.895   $73.90   $73.90   $73.90  
  Purchase #4 $102.78   4.620%   $107.5284   $107.53   $107.55   $107.50  
  Purchase #5 $128.49   12.259%   $144.2416   $144.24   $144.25   $144.20  
  Purchase #6 $229.38   13.761%   $260.945   $260.95   $260.95   $260.90  
  Purchase #7 $36.96   9.597%   $40.5071   $40.51   $40.50   $40.50  
  7 purchase totals = $973.61       $1,075.8535   $1,075.87   $1,075.85   $1,075.80