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 $242.40   11.166%   $269.4664   $269.47   $269.45   $269.50  
  Purchase #2 $51.61   3.718%   $53.5289   $53.53   $53.55   $53.50  
  Purchase #3 $32.98   4.212%   $34.3691   $34.37   $34.35   $34.40  
  Purchase #4 $207.85   6.250%   $220.8406   $220.84   $220.85   $220.80  
  Purchase #5 $125.72   7.350%   $134.9604   $134.96   $134.95   $135.00  
  Purchase #6 $214.89   11.452%   $239.4992   $239.50   $239.50   $239.50  
  Purchase #7 $56.91   13.658%   $64.6828   $64.68   $64.70   $64.70  
  7 purchase totals = $932.36       $1,017.3474   $1,017.35   $1,017.35   $1,017.40