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 $127.32   6.728%   $135.8861   $135.89   $135.90   $135.90  
  Purchase #2 $245.29   12.676%   $276.383   $276.38   $276.40   $276.40  
  Purchase #3 $122.60   13.436%   $139.0725   $139.07   $139.05   $139.10  
  Purchase #4 $19.81   8.272%   $21.4487   $21.45   $21.45   $21.40  
  Purchase #5 $140.50   13.184%   $159.0235   $159.02   $159.00   $159.00  
  Purchase #6 $115.50   10.339%   $127.4415   $127.44   $127.45   $127.40  
  Purchase #7 $174.68   3.769%   $181.2637   $181.26   $181.25   $181.30  
  7 purchase totals = $945.70       $1,040.5190   $1,040.51   $1,040.50   $1,040.50