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 $16.59   6.244%   $17.6259   $17.63   $17.65   $17.60  
  Purchase #2 $133.39   4.479%   $139.3645   $139.36   $139.35   $139.40  
  Purchase #3 $145.64   8.563%   $158.1112   $158.11   $158.10   $158.10  
  Purchase #4 $89.70   11.606%   $100.1106   $100.11   $100.10   $100.10  
  Purchase #5 $26.93   7.685%   $28.9996   $29.00   $29.00   $29.00  
  Purchase #6 $99.77   3.500%   $103.262   $103.26   $103.25   $103.30  
  Purchase #7 $26.95   12.150%   $30.2244   $30.22   $30.20   $30.20  
  7 purchase totals = $538.97       $577.6982   $577.69   $577.65   $577.70