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 $230.71   12.306%   $259.1012   $259.10   $259.10   $259.10  
  Purchase #2 $168.57   6.103%   $178.8578   $178.86   $178.85   $178.90  
  Purchase #3 $118.70   9.189%   $129.6073   $129.61   $129.60   $129.60  
  Purchase #4 $239.81   7.868%   $258.6783   $258.68   $258.70   $258.70  
  Purchase #5 $212.87   9.606%   $233.3183   $233.32   $233.30   $233.30  
  Purchase #6 $80.87   3.713%   $83.8727   $83.87   $83.85   $83.90  
  Purchase #7 $161.84   11.261%   $180.0648   $180.06   $180.05   $180.10  
  7 purchase totals = $1,213.37       $1,323.5004   $1,323.50   $1,323.45   $1,323.60