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 $220.79   13.239%   $250.0204   $250.02   $250.00   $250.00  
  Purchase #2 $146.71   10.129%   $161.5703   $161.57   $161.55   $161.60  
  Purchase #3 $110.33   12.769%   $124.418   $124.42   $124.40   $124.40  
  Purchase #4 $184.51   4.606%   $193.0085   $193.01   $193.00   $193.00  
  Purchase #5 $25.39   7.849%   $27.3829   $27.38   $27.40   $27.40  
  Purchase #6 $108.98   9.653%   $119.4998   $119.50   $119.50   $119.50  
  Purchase #7 $62.72   9.210%   $68.4965   $68.50   $68.50   $68.50  
  7 purchase totals = $859.43       $944.3964   $944.40   $944.35   $944.40