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 $225.70   12.495%   $253.9012   $253.90   $253.90   $253.90  
  Purchase #2 $66.97   3.149%   $69.0789   $69.08   $69.10   $69.10  
  Purchase #3 $69.54   6.286%   $73.9113   $73.91   $73.90   $73.90  
  Purchase #4 $104.44   4.868%   $109.5241   $109.52   $109.50   $109.50  
  Purchase #5 $22.94   11.584%   $25.5974   $25.60   $25.60   $25.60  
  Purchase #6 $162.99   7.365%   $174.9942   $174.99   $175.00   $175.00  
  Purchase #7 $248.74   11.667%   $277.7605   $277.76   $277.75   $277.80  
  7 purchase totals = $901.32       $984.7676   $984.76   $984.75   $984.80