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 $56.38   6.491%   $60.0396   $60.04   $60.05   $60.00  
  Purchase #2 $104.54   13.671%   $118.8317   $118.83   $118.85   $118.80  
  Purchase #3 $99.91   8.496%   $108.3984   $108.40   $108.40   $108.40  
  Purchase #4 $37.39   5.423%   $39.4177   $39.42   $39.40   $39.40  
  Purchase #5 $239.70   13.422%   $271.8725   $271.87   $271.85   $271.90  
  Purchase #6 $39.52   4.281%   $41.2119   $41.21   $41.20   $41.20  
  Purchase #7 $173.69   13.705%   $197.4942   $197.49   $197.50   $197.50  
  7 purchase totals = $751.13       $837.2660   $837.26   $837.25   $837.20