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 $61.33   6.755%   $65.4728   $65.47   $65.45   $65.50  
  Purchase #2 $232.59   8.332%   $251.9694   $251.97   $251.95   $252.00  
  Purchase #3 $124.64   11.794%   $139.34   $139.34   $139.35   $139.30  
  Purchase #4 $123.72   8.639%   $134.4082   $134.41   $134.40   $134.40  
  Purchase #5 $36.46   13.498%   $41.3814   $41.38   $41.40   $41.40  
  Purchase #6 $35.40   13.253%   $40.0916   $40.09   $40.10   $40.10  
  Purchase #7 $15.98   8.561%   $17.348   $17.35   $17.35   $17.30  
  7 purchase totals = $630.12       $690.0114   $690.01   $690.00   $690.00