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 $120.85   12.543%   $136.0082   $136.01   $136.00   $136.00  
  Purchase #2 $142.58   11.622%   $159.1506   $159.15   $159.15   $159.20  
  Purchase #3 $214.59   8.176%   $232.1349   $232.13   $232.15   $232.10  
  Purchase #4 $12.51   6.577%   $13.3328   $13.33   $13.35   $13.30  
  Purchase #5 $54.71   9.624%   $59.9753   $59.98   $60.00   $60.00  
  Purchase #6 $111.97   13.744%   $127.3592   $127.36   $127.35   $127.40  
  Purchase #7 $67.27   13.351%   $76.2512   $76.25   $76.25   $76.30  
  7 purchase totals = $724.48       $804.2122   $804.21   $804.25   $804.30