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 $241.97   6.282%   $257.1706   $257.17   $257.15   $257.20  
  Purchase #2 $168.93   3.524%   $174.8831   $174.88   $174.90   $174.90  
  Purchase #3 $98.55   3.179%   $101.6829   $101.68   $101.70   $101.70  
  Purchase #4 $219.46   11.449%   $244.586   $244.59   $244.60   $244.60  
  Purchase #5 $164.76   12.748%   $185.7636   $185.76   $185.75   $185.80  
  Purchase #6 $195.53   9.685%   $214.4671   $214.47   $214.45   $214.50  
  Purchase #7 $32.48   6.775%   $34.6805   $34.68   $34.70   $34.70  
  7 purchase totals = $1,121.68       $1,213.2338   $1,213.23   $1,213.25   $1,213.40