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 $6.74   7.816%   $7.2668   $7.27   $7.25   $7.30  
  Purchase #2 $230.45   8.207%   $249.363   $249.36   $249.35   $249.40  
  Purchase #3 $201.98   9.432%   $221.0308   $221.03   $221.05   $221.00  
  Purchase #4 $20.79   5.126%   $21.8557   $21.86   $21.85   $21.90  
  Purchase #5 $100.98   13.215%   $114.3245   $114.32   $114.30   $114.30  
  Purchase #6 $61.91   9.144%   $67.5711   $67.57   $67.55   $67.60  
  Purchase #7 $162.59   9.536%   $178.0946   $178.09   $178.10   $178.10  
  7 purchase totals = $785.44       $859.5065   $859.50   $859.45   $859.60