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 $226.30   13.792%   $257.5113   $257.51   $257.50   $257.50  
  Purchase #2 $131.31   7.392%   $141.0164   $141.02   $141.00   $141.00  
  Purchase #3 $104.38   10.686%   $115.534   $115.53   $115.55   $115.50  
  Purchase #4 $205.49   7.599%   $221.1052   $221.11   $221.10   $221.10  
  Purchase #5 $226.43   10.825%   $250.941   $250.94   $250.95   $250.90  
  Purchase #6 $97.56   13.772%   $110.996   $111.00   $111.00   $111.00  
  Purchase #7 $43.99   5.225%   $46.2885   $46.29   $46.30   $46.30  
  7 purchase totals = $1,035.46       $1,143.3924   $1,143.40   $1,143.40   $1,143.30