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 $247.67   8.753%   $269.3486   $269.35   $269.35   $269.30  
  Purchase #2 $7.99   4.552%   $8.3537   $8.35   $8.35   $8.40  
  Purchase #3 $124.45   8.781%   $135.378   $135.38   $135.40   $135.40  
  Purchase #4 $135.27   11.422%   $150.7205   $150.72   $150.70   $150.70  
  Purchase #5 $65.89   6.157%   $69.9468   $69.95   $69.95   $69.90  
  Purchase #6 $157.67   5.233%   $165.9209   $165.92   $165.90   $165.90  
  Purchase #7 $54.48   11.734%   $60.8727   $60.87   $60.85   $60.90  
  7 purchase totals = $793.42       $860.5412   $860.54   $860.50   $860.50