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 $249.89   12.858%   $282.0209   $282.02   $282.00   $282.00  
  Purchase #2 $108.87   8.355%   $117.9661   $117.97   $117.95   $118.00  
  Purchase #3 $87.25   9.707%   $95.7194   $95.72   $95.70   $95.70  
  Purchase #4 $240.63   4.159%   $250.6378   $250.64   $250.65   $250.60  
  Purchase #5 $71.79   10.113%   $79.0501   $79.05   $79.05   $79.10  
  Purchase #6 $159.79   13.252%   $180.9654   $180.97   $180.95   $181.00  
  Purchase #7 $203.47   5.493%   $214.6466   $214.65   $214.65   $214.60  
  7 purchase totals = $1,121.69       $1,221.0063   $1,221.02   $1,220.95   $1,221.00