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 $204.44   6.198%   $217.1112   $217.11   $217.10   $217.10  
  Purchase #2 $43.64   13.862%   $49.6894   $49.69   $49.70   $49.70  
  Purchase #3 $84.71   8.509%   $91.918   $91.92   $91.90   $91.90  
  Purchase #4 $189.99   12.555%   $213.8432   $213.84   $213.85   $213.80  
  Purchase #5 $203.65   6.678%   $217.2497   $217.25   $217.25   $217.20  
  Purchase #6 $242.30   6.432%   $257.8847   $257.88   $257.90   $257.90  
  Purchase #7 $242.25   3.277%   $250.1885   $250.19   $250.20   $250.20  
  7 purchase totals = $1,210.98       $1,297.8847   $1,297.88   $1,297.90   $1,297.80