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 $235.42   13.260%   $266.6367   $266.64   $266.65   $266.60  
  Purchase #2 $171.99   9.832%   $188.9001   $188.90   $188.90   $188.90  
  Purchase #3 $214.65   3.870%   $222.957   $222.96   $222.95   $223.00  
  Purchase #4 $228.45   8.384%   $247.6032   $247.60   $247.60   $247.60  
  Purchase #5 $130.67   13.662%   $148.5221   $148.52   $148.50   $148.50  
  Purchase #6 $15.74   4.361%   $16.4264   $16.43   $16.45   $16.40  
  Purchase #7 $59.32   12.339%   $66.6395   $66.64   $66.65   $66.60  
  7 purchase totals = $1,056.24       $1,157.6850   $1,157.69   $1,157.70   $1,157.60