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 $100.38   8.757%   $109.1703   $109.17   $109.15   $109.20  
  Purchase #2 $18.84   3.491%   $19.4977   $19.50   $19.50   $19.50  
  Purchase #3 $11.91   11.106%   $13.2327   $13.23   $13.25   $13.20  
  Purchase #4 $161.66   12.840%   $182.4171   $182.42   $182.40   $182.40  
  Purchase #5 $176.50   11.688%   $197.1293   $197.13   $197.15   $197.10  
  Purchase #6 $71.42   13.328%   $80.9389   $80.94   $80.95   $80.90  
  Purchase #7 $27.89   7.410%   $29.9566   $29.96   $29.95   $30.00  
  7 purchase totals = $568.60       $632.3426   $632.35   $632.35   $632.30