Initializing and populating a Python dictionary, key -> List
line.strip() does not modify the variable, it returns a modified string. But you can chain it together with the split. I would also jchoose a better variable name than rec. As J.F.Sebastian suggested in the comments, you could go with items:

for line in f:
    items = line.strip().split('\t')


Your code is the perfect place for a collections.defaultdict. You can give it a type which it will use when the key is not defined. It makes implementing a counter a lot easier (just pass it int, whose default constructor returns an int with value 0) or, give it list and it will give you an empty list:

from collections import defaultdict

value_dict = defaultdict(list)

with open("file.txt") as f:
    for line in f:
        items = line.strip().split('\t')
        key, value = items[0], items[5]
        value_dict[key].append(value)


I also added the with..as construct in there, in case you are not yet using it.

In python 3.x I would use the extended iterable unpacking:

key, *other_vals, value = line.strip().split('\t')