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 $12.90   6.780%   $13.7746   $13.77   $13.75   $13.80  
  Purchase #2 $109.50   10.620%   $121.1289   $121.13   $121.15   $121.10  
  Purchase #3 $187.87   3.103%   $193.6996   $193.70   $193.70   $193.70  
  Purchase #4 $239.96   11.638%   $267.8865   $267.89   $267.90   $267.90  
  Purchase #5 $32.75   12.406%   $36.813   $36.81   $36.80   $36.80  
  Purchase #6 $167.86   3.481%   $173.7032   $173.70   $173.70   $173.70  
  Purchase #7 $11.84   6.137%   $12.5666   $12.57   $12.55   $12.60  
  7 purchase totals = $762.68       $819.5724   $819.57   $819.55   $819.60