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 $17.99   4.598%   $18.8172   $18.82   $18.80   $18.80  
  Purchase #2 $96.95   3.130%   $99.9845   $99.98   $100.00   $100.00  
  Purchase #3 $122.74   12.859%   $138.5231   $138.52   $138.50   $138.50  
  Purchase #4 $74.38   9.583%   $81.5078   $81.51   $81.50   $81.50  
  Purchase #5 $44.86   5.502%   $47.3282   $47.33   $47.35   $47.30  
  Purchase #6 $169.68   5.843%   $179.5944   $179.59   $179.60   $179.60  
  Purchase #7 $51.99   12.561%   $58.5205   $58.52   $58.50   $58.50  
  7 purchase totals = $578.59       $624.2757   $624.27   $624.25   $624.20