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 $16.40   9.715%   $17.9933   $17.99   $18.00   $18.00  
  Purchase #2 $99.90   3.320%   $103.2167   $103.22   $103.20   $103.20  
  Purchase #3 $140.35   6.851%   $149.9654   $149.97   $149.95   $150.00  
  Purchase #4 $78.99   9.870%   $86.7863   $86.79   $86.80   $86.80  
  Purchase #5 $130.77   6.319%   $139.0334   $139.03   $139.05   $139.00  
  Purchase #6 $26.95   13.672%   $30.6346   $30.63   $30.65   $30.60  
  Purchase #7 $80.30   5.774%   $84.9365   $84.94   $84.95   $84.90  
  7 purchase totals = $573.66       $612.5662   $612.57   $612.60   $612.50