It's safe to call it whatever you want to call it, so long as the story is strong, and this goes for the stories that use that primitive as well.

In RTK III, Heisig takes rice plus sunglasses (such as in 隣 neighboring) and calls it "shoeshine", admitting that the meaning was chosen purely arbitrarily.

a note on changing the assigned meanings of the primitives:

some of the meanings heisig has assigned the primitives are based on etymology, but many are totally arbitrary (like "shoeshine" mentioned above).  a lot of the arbitrary assignations seem to be based on some assigned meaning that arises easily from visual similarity or the combination of constituent primitives.  others seem to get assigned an arbitrary meaning that makes them easy to use in the specific set of kanji they will appear in later.

since many of heisig's assigned meanings are arbitrary, i don't see the harm in re-assigning your own arbitrary meanings.  however, if you are willing to spend a little extra time to do it right, here are some tips that might make this easier in the long run.

before assigning a new meaning to a troublesome primitive:
--check Index IV in the book to make sure you are not assigning a meaning that will show up later as a kanji keyword or primitive meaning.
--use some software that searches the EDICT kanji dictionary files by radical (Wakan and the like can do this quickly and efficiently) this should show you the kanji and keywords you will have to make stories for using your new meaning.  if it looks like your proposed meaning is going to make this a pain in the ass for most of those kanji, you might reconsider.  try to use a new meaning that is going to mesh well with a large number of those kanji keywords.
--if you can see that heisig's primitive meaning is based fairly firmly on etymology (not sure, but i suspect "piggy bank" is not one of these) i suggest leaving it as is and working with it as best as possible.  if you are serious about studying kanji, i believe this will probably be of service in the long run.
--along similar lines to this, you can also use Wakan or some search software to see if the primitive in question shows up as a non-RTK kanji in its own right.  but be careful, i have occasionally found this very helpful, and occasionally found it difficult and confusing.  where it has been difficult is when the primitive shows up as a kanji that has a meaning very close to some other RTK kanji.

i have frequently used these methods to successfully change the meanings of primitives to suit my own style, or at least get a better feel for the most effective way to image them for the task ahead.

anyways, none of these tips are meant to be applied to converting some primitive into a character based on its heisig assigned meaning: Shrek for "fingers", Black Widow for "thread", etc.  though i generally avoid this myself, i think for many it's probably the best way to go for making difficult, static, or abstract primitives more easily incorporated into stories.

Ramchip wrote:

Actually, I find it easier to remember piggy bank using the true etymological sense of "rise up". It explains most meanings better, such as vapour which is water + rise up. You can look up stuff on kanji networks to find the true meaning.

THANK YOU, THANK YOU, THANK YOU! It was totally impossible for me to recall the stories featuring the stupid piggy bank and I was desperate for the etymological origin of the primitive.

I don't understand why Heisig had to complicate things with such unmanageable keywords.