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 $17.31   3.287%   $17.879   $17.88   $17.90   $17.90  
  Purchase #2 $29.64   4.509%   $30.9765   $30.98   $31.00   $31.00  
  Purchase #3 $77.42   4.543%   $80.9372   $80.94   $80.95   $80.90  
  Purchase #4 $101.26   13.599%   $115.0303   $115.03   $115.05   $115.00  
  Purchase #5 $8.94   9.803%   $9.8164   $9.82   $9.80   $9.80  
  Purchase #6 $134.52   11.409%   $149.8674   $149.87   $149.85   $149.90  
  Purchase #7 $195.54   3.568%   $202.5169   $202.52   $202.50   $202.50  
  7 purchase totals = $564.63       $607.0237   $607.04   $607.05   $607.00