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 $18.75   9.619%   $20.5536   $20.55   $20.55   $20.60  
  Purchase #2 $206.88   5.418%   $218.0888   $218.09   $218.10   $218.10  
  Purchase #3 $202.85   6.723%   $216.4876   $216.49   $216.50   $216.50  
  Purchase #4 $123.87   8.604%   $134.5278   $134.53   $134.55   $134.50  
  Purchase #5 $91.25   13.715%   $103.7649   $103.76   $103.75   $103.80  
  Purchase #6 $174.41   8.116%   $188.5651   $188.57   $188.55   $188.60  
  Purchase #7 $90.92   12.158%   $101.9741   $101.97   $101.95   $102.00  
  7 purchase totals = $908.93       $983.9619   $983.96   $983.95   $984.10