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 $19.83   8.742%   $21.5635   $21.56   $21.55   $21.60  
  Purchase #2 $21.39   9.715%   $23.468   $23.47   $23.45   $23.50  
  Purchase #3 $211.57   11.147%   $235.1537   $235.15   $235.15   $235.20  
  Purchase #4 $17.69   12.318%   $19.8691   $19.87   $19.85   $19.90  
  Purchase #5 $90.74   9.546%   $99.402   $99.40   $99.40   $99.40  
  Purchase #6 $83.42   7.847%   $89.966   $89.97   $89.95   $90.00  
  Purchase #7 $108.59   10.566%   $120.0636   $120.06   $120.05   $120.10  
  7 purchase totals = $553.23       $609.4859   $609.48   $609.40   $609.70