That isn't always an option though. First of all, you may not want to tip your hand that you aren't playing field spells. Secondly, it could be considered an issue for the opposing player to put any card in a zone that it was designated to go to. What you are suggesting is tantamount to saying an RFP monster can be put in a spell or trap zone "because you weren't planning on using all 5 anyway". But if that monster is a crystal beast ...
There also isn't always room to create another visible RFP zone on the table that doesn't create confusion. So while I would advise against it myself, if both players agree that their RFP cards are put in the grave sideways, or are placed upside down in the correct order underneat the grave then that's fine by me. I'm aware that the RFP zone should be face-up, but since I'm of the opinion you should always ask before grabbing an opponents graved or RFP'ed cards, its not a big deal, since you can just ask to see the RFP zone at any time.
If there is room to create an RFP zone outside of the playing field that should always be pursued in a fashion that does not create confusion. If there is not, I feel that an agreement made between two players in the presence of a judge should be enough to prevent further conflict because of practices like placing cards sideways.
On the initial issue I have to agree that placing the frog sideways is a no-no. If you are too stupid to remember you have a frog in your graveyard, spend more time studying and less time playing TCG's.