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 $51.78   6.694%   $55.2462   $55.25   $55.25   $55.20  
  Purchase #2 $230.82   11.195%   $256.6603   $256.66   $256.65   $256.70  
  Purchase #3 $221.29   9.207%   $241.6642   $241.66   $241.65   $241.70  
  Purchase #4 $239.90   9.415%   $262.4866   $262.49   $262.50   $262.50  
  Purchase #5 $128.95   4.591%   $134.8701   $134.87   $134.85   $134.90  
  Purchase #6 $23.94   12.183%   $26.8566   $26.86   $26.85   $26.90  
  Purchase #7 $227.48   11.621%   $253.9155   $253.92   $253.90   $253.90  
  7 purchase totals = $1,124.16       $1,231.6995   $1,231.71   $1,231.65   $1,231.80