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 $137.63   12.868%   $155.3402   $155.34   $155.35   $155.30  
  Purchase #2 $242.78   9.197%   $265.1085   $265.11   $265.10   $265.10  
  Purchase #3 $15.77   6.161%   $16.7416   $16.74   $16.75   $16.70  
  Purchase #4 $250.33   7.577%   $269.2975   $269.30   $269.30   $269.30  
  Purchase #5 $80.53   4.401%   $84.0741   $84.07   $84.05   $84.10  
  Purchase #6 $87.31   7.193%   $93.5902   $93.59   $93.60   $93.60  
  Purchase #7 $13.61   10.117%   $14.9869   $14.99   $15.00   $15.00  
  7 purchase totals = $827.96       $899.1390   $899.14   $899.15   $899.10