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 $55.67   5.127%   $58.5242   $58.52   $58.50   $58.50  
  Purchase #2 $101.88   11.558%   $113.6553   $113.66   $113.65   $113.70  
  Purchase #3 $123.57   9.686%   $135.539   $135.54   $135.55   $135.50  
  Purchase #4 $133.36   7.336%   $143.1433   $143.14   $143.15   $143.10  
  Purchase #5 $11.59   5.389%   $12.2146   $12.21   $12.20   $12.20  
  Purchase #6 $174.72   11.623%   $195.0277   $195.03   $195.05   $195.00  
  Purchase #7 $229.74   4.166%   $239.311   $239.31   $239.30   $239.30  
  7 purchase totals = $830.53       $897.4151   $897.41   $897.40   $897.30