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 $9.42   4.768%   $9.8691   $9.87   $9.85   $9.90  
  Purchase #2 $24.36   10.645%   $26.9531   $26.95   $26.95   $27.00  
  Purchase #3 $92.65   13.868%   $105.4987   $105.50   $105.50   $105.50  
  Purchase #4 $95.54   11.186%   $106.2271   $106.23   $106.25   $106.20  
  Purchase #5 $135.30   9.440%   $148.0723   $148.07   $148.05   $148.10  
  Purchase #6 $197.60   10.319%   $217.9903   $217.99   $218.00   $218.00  
  Purchase #7 $25.44   12.580%   $28.6404   $28.64   $28.65   $28.60  
  7 purchase totals = $580.31       $643.2510   $643.25   $643.25   $643.30