allow means to permit, and allow for is more like to make something
possible, to enable, to make a provision for, but I'm still in doubt
when I have to decide whether to use the preposition for or not.
Refining that characterizaton somewhat, consider also that the forms, "allow..." and "allow for..." are suggestive of verb (or verb and helping word) moods. To say, allow a thing most likely reflects an imperative verb mood (in the sense of a command albeit a passive and/or implied command. To allow a thing could also be indicative, a simple statement of happenstance...especially in past tense: e.g., "... you allowed the sink to overflow." (Obviously, you don't normally allow for a sink to overflow....unless, perhaps, you are installing a floor drain; but that, still would not entail indicative mood, as demonstrated next.)
Allowing for (a thing), on the other hand, explicitly connotes that the allowance more than equals the need; that there is doubt as to sufficiency of that which will or might be neeeded to be allowed. Such doubt sets the mood of the phrase, allow for, as subjunctive. (In the floor drain example, the indefinite capacity of the floor drain reveals uncertainty as to how much overflow will actually need to be accommodated...so, still, subjunctive.
So all you need to do is figure out the mood in which allow is used, then modify, or not, accordingly.
The prevailing verb moods are: indicative, imperative, subjunctive, and infinitive:
moods of verbs