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 $249.83   10.422%   $275.8673   $275.87   $275.85   $275.90  
  Purchase #2 $89.32   10.492%   $98.6915   $98.69   $98.70   $98.70  
  Purchase #3 $25.43   3.322%   $26.2748   $26.27   $26.25   $26.30  
  Purchase #4 $105.89   10.798%   $117.324   $117.32   $117.30   $117.30  
  Purchase #5 $184.73   4.836%   $193.6635   $193.66   $193.65   $193.70  
  Purchase #6 $229.76   9.559%   $251.7228   $251.72   $251.70   $251.70  
  Purchase #7 $35.99   5.451%   $37.9518   $37.95   $37.95   $38.00  
  7 purchase totals = $920.95       $1,001.4957   $1,001.48   $1,001.40   $1,001.60