Memory wipes seem to me to be a perfectly acceptable way of maintaining the integrity of how they behave in ANH. Uncle Owen tells Luke to have it done after he reveals that they have mentioned Obi-Wan. This tells us:
(a) the procedure is standard practice.
(b) it is provided as a service, like for any other kind of machinery.
This allows us to have a little fun with the droids at their expense because, unlike your classic immortals, they are almost entirely at the mercy of their organic masters.
They could have their memories erased at the beginning of Episode I and then again when they are given to Captain Antilles. This means that the droids don't know what we know at the beginning of ANH. I don't think that this would in any way impair their role as companions for us. If we establish that the procedure is standard for droids when changing hands, then Threepio and Artoo would be quite used to it. It gives us a nice mystery as to how many owners the droids have had since being built and leaves us unsure as to just how old they really are. It might be fun to have the droids allude to their most recent owner and their experiences with him or her before their memories are erased in Episode I.
After all, what is the SW experience without Threepio and Artoo?!