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 $99.25   10.221%   $109.3943   $109.39   $109.40   $109.40  
  Purchase #2 $249.88   3.209%   $257.8986   $257.90   $257.90   $257.90  
  Purchase #3 $67.58   9.241%   $73.8251   $73.83   $73.85   $73.80  
  Purchase #4 $69.39   7.620%   $74.6775   $74.68   $74.70   $74.70  
  Purchase #5 $76.88   3.300%   $79.417   $79.42   $79.40   $79.40  
  Purchase #6 $180.56   7.563%   $194.2158   $194.22   $194.20   $194.20  
  Purchase #7 $75.66   4.711%   $79.2243   $79.22   $79.20   $79.20  
  7 purchase totals = $819.20       $868.6526   $868.66   $868.65   $868.60