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 $76.49   10.451%   $84.484   $84.48   $84.50   $84.50  
  Purchase #2 $14.99   3.314%   $15.4868   $15.49   $15.50   $15.50  
  Purchase #3 $178.74   6.701%   $190.7174   $190.72   $190.70   $190.70  
  Purchase #4 $113.37   12.124%   $127.115   $127.12   $127.10   $127.10  
  Purchase #5 $125.32   6.408%   $133.3505   $133.35   $133.35   $133.40  
  Purchase #6 $118.77   4.459%   $124.066   $124.07   $124.05   $124.10  
  Purchase #7 $162.35   5.537%   $171.3393   $171.34   $171.35   $171.30  
  7 purchase totals = $790.03       $846.5590   $846.57   $846.55   $846.60