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 $27.91   7.686%   $30.0552   $30.06   $30.05   $30.10  
  Purchase #2 $60.66   11.690%   $67.7512   $67.75   $67.75   $67.80  
  Purchase #3 $81.40   6.294%   $86.5233   $86.52   $86.50   $86.50  
  Purchase #4 $78.35   12.615%   $88.2339   $88.23   $88.25   $88.20  
  Purchase #5 $134.83   3.440%   $139.4682   $139.47   $139.45   $139.50  
  Purchase #6 $89.68   10.554%   $99.1448   $99.14   $99.15   $99.10  
  Purchase #7 $168.58   5.146%   $177.2551   $177.26   $177.25   $177.30  
  7 purchase totals = $641.41       $688.4317   $688.43   $688.40   $688.50