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 $14.69   6.450%   $15.6375   $15.64   $15.65   $15.60  
  Purchase #2 $128.28   7.639%   $138.0793   $138.08   $138.10   $138.10  
  Purchase #3 $241.49   3.179%   $249.167   $249.17   $249.15   $249.20  
  Purchase #4 $132.37   5.806%   $140.0554   $140.06   $140.05   $140.10  
  Purchase #5 $32.65   12.567%   $36.7531   $36.75   $36.75   $36.80  
  Purchase #6 $114.50   3.762%   $118.8075   $118.81   $118.80   $118.80  
  Purchase #7 $189.77   11.839%   $212.2369   $212.24   $212.25   $212.20  
  7 purchase totals = $853.75       $910.7367   $910.75   $910.75   $910.80