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 $250.81   12.136%   $281.2483   $281.25   $281.25   $281.20  
  Purchase #2 $246.78   12.145%   $276.7514   $276.75   $276.75   $276.80  
  Purchase #3 $224.95   6.559%   $239.7045   $239.70   $239.70   $239.70  
  Purchase #4 $84.74   3.627%   $87.8135   $87.81   $87.80   $87.80  
  Purchase #5 $14.68   5.407%   $15.4737   $15.47   $15.45   $15.50  
  Purchase #6 $213.66   8.761%   $232.3788   $232.38   $232.40   $232.40  
  Purchase #7 $227.96   4.199%   $237.532   $237.53   $237.55   $237.50  
  7 purchase totals = $1,263.58       $1,370.9022   $1,370.89   $1,370.90   $1,370.90