Many people within the umbrella of Heathenry have the idea that the worship of the dead was an integral part of the historical religion of the peoples of Northern Europe. To some extent, this is true, but almost never in the way that modern Heathens seem to think it was.
Many modern Heathens will, for example, keep a shrine to their ancestors somewhere in their home. Pictures of grandfathers and more historical figures adorn a table or shelf, or even a full-blown altar. Some will perform rituals in honor of those ancestors, usually variations on the same sorts of rituals that are used to honor the Gods, land-wights, etc. Unfortunately, this is not a practice attested to at all in either the written lore or the living folklore of Scandinavia, Great Britain, or the northwest Continent.
Some well-respected Heathen scholars have approached me on this topic, and are certain that they have seen attested references to such worship somewhere. When we try to track down these elusive references, however, they seem to have never existed. The desire (and perhaps the need) for such a practice seems to play tricks on the memory of just what has, or has not, been actually read. This is by no means a failing on anyone's part; many's the time I could swear I read something, only to find that I either misremembered what I thought I had read, or just couldn't find it at all when I try to revisit a particular subject.
This is not to say that the dead were never venerated; far from it. The practice of mound-sitting is well-attested to in the lore, for example. But the mounds what were involved were those of kings or other influential members of the community (particularly in Iceland, where there were no kings, but fallen goðar seem to have taken their place in some instances). But by no means was every fallen ancestor so honored.
Also, we have the minne, or memorial toast. This is a toast, made during sumbl, in honor of an ancestor. But that is something done to both honor the ancestor and the person making the toast (by virtue of connecting that person making the toast to an ancestor of great renown, implying that such renown reflects positively on the person making the minne toast). Fine and good, and I myself have made such toasts, and will again.
There is also the singular ritual of the arvel; a feast in honor of a fallen famly member. When this was a head of the household, his successor would ceremonially assume the headship of the family as a part of the rite. Doubtless many minne toasts were made in honor of the fallen. However, this is a one-time event, not a regular ritual. Its primary purpose was to ceremonially provide continuity between the dead relative and the new head of the family. It is, essentially, a special form of sumbl, and in no way resembles a regular offering to a dead relative at some family shrine.
Ancestor worship, in the form of offerings made regularly at some sort of household altar or shrine, is simply not a practice supported in the lore, as far as I can see. If someone has a reference to such a thing, I would greatly appreciate it if you could send it my way, as I am more than happy to change my attitudes on such things when presented with new evidence.
Does this mean that modern Ásatrúar, for example, must abandon the practice of having a household shrine dedicated to their ancestors, and making regular offerings to them? Of course not. But it does mean that they need to understand and accept the fact that doing so is not an historical practice. Different branches of Heathenry pay more or less attention to such things, and although my own Þéodish Belief rests squarely on the more historical end of the spectrum (and I emphasize historicity more than most), such practices must be left to the conscience of the individual.
But personally, I see no need to insert something where it was not before, as far as we know.