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 $21.48   6.327%   $22.839   $22.84   $22.85   $22.80  
  Purchase #2 $233.85   9.323%   $255.6518   $255.65   $255.65   $255.70  
  Purchase #3 $216.85   6.764%   $231.5177   $231.52   $231.50   $231.50  
  Purchase #4 $38.55   9.716%   $42.2955   $42.30   $42.30   $42.30  
  Purchase #5 $180.30   8.679%   $195.9482   $195.95   $195.95   $195.90  
  Purchase #6 $237.76   4.700%   $248.9347   $248.93   $248.95   $248.90  
  Purchase #7 $23.38   4.571%   $24.4487   $24.45   $24.45   $24.40  
  7 purchase totals = $952.17       $1,021.6356   $1,021.64   $1,021.65   $1,021.50