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 $249.46   7.446%   $268.0348   $268.03   $268.05   $268.00  
  Purchase #2 $110.53   8.209%   $119.6034   $119.60   $119.60   $119.60  
  Purchase #3 $110.56   10.388%   $122.045   $122.05   $122.05   $122.00  
  Purchase #4 $178.85   8.779%   $194.5512   $194.55   $194.55   $194.60  
  Purchase #5 $16.38   4.371%   $17.096   $17.10   $17.10   $17.10  
  Purchase #6 $101.98   11.518%   $113.7261   $113.73   $113.75   $113.70  
  Purchase #7 $147.43   11.638%   $164.5879   $164.59   $164.60   $164.60  
  7 purchase totals = $915.19       $999.6444   $999.65   $999.70   $999.60