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 $19.40   3.540%   $20.0868   $20.09   $20.10   $20.10  
  Purchase #2 $163.40   8.455%   $177.2155   $177.22   $177.20   $177.20  
  Purchase #3 $199.58   6.843%   $213.2373   $213.24   $213.25   $213.20  
  Purchase #4 $164.59   11.501%   $183.5195   $183.52   $183.50   $183.50  
  Purchase #5 $114.93   11.514%   $128.163   $128.16   $128.15   $128.20  
  Purchase #6 $242.85   4.124%   $252.8651   $252.87   $252.85   $252.90  
  Purchase #7 $23.84   4.186%   $24.8379   $24.84   $24.85   $24.80  
  7 purchase totals = $928.59       $999.9251   $999.94   $999.90   $999.90