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 $235.53   12.859%   $265.8168   $265.82   $265.80   $265.80  
  Purchase #2 $93.65   6.689%   $99.9142   $99.91   $99.90   $99.90  
  Purchase #3 $65.79   12.127%   $73.7684   $73.77   $73.75   $73.80  
  Purchase #4 $105.43   11.857%   $117.9308   $117.93   $117.95   $117.90  
  Purchase #5 $237.45   9.242%   $259.3951   $259.40   $259.40   $259.40  
  Purchase #6 $194.48   6.685%   $207.481   $207.48   $207.50   $207.50  
  Purchase #7 $153.49   3.276%   $158.5183   $158.52   $158.50   $158.50  
  7 purchase totals = $1,085.82       $1,182.8246   $1,182.83   $1,182.80   $1,182.80