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 $246.79   6.239%   $262.1872   $262.19   $262.20   $262.20  
  Purchase #2 $192.54   5.390%   $202.9179   $202.92   $202.90   $202.90  
  Purchase #3 $201.58   4.356%   $210.3608   $210.36   $210.35   $210.40  
  Purchase #4 $19.73   3.330%   $20.387   $20.39   $20.40   $20.40  
  Purchase #5 $236.74   5.428%   $249.5902   $249.59   $249.60   $249.60  
  Purchase #6 $175.67   6.202%   $186.5651   $186.57   $186.55   $186.60  
  Purchase #7 $145.68   9.312%   $159.2457   $159.25   $159.25   $159.20  
  7 purchase totals = $1,218.73       $1,291.2539   $1,291.27   $1,291.25   $1,291.30