RankWatch will monitor the spells cast by you and other level 80 members of your party or raid and warn them if they are using a downranked spell.

While downranked spells were formerly used to save mana, this use is no longer supported, and while it is occasionally intended, the use of downranked spells is generally a mistake. Sometimes this happens when people forget to train at level 80, sometimes it happens when they spend talent points to obtain a spell which is not a base skill for the class (for example, a Death Knight's Frost Strike our Scourge Strike) but fail to train the ranks for the newly talented spell. In my experience, the most common cause is training while dual specced. For example, if you level up as a feral druid, you will train new ranks of Rejuvenation etc, but the new ranks will not appear on your bars when you change to the Resto spec.

There are a few special cases - Life Tap and Health Funnel are not reported, since Life Tap is often used by warlocks to proc the Glyph and stack trinkets, and Health Funnel costs less health at lower ranks. Polymorph is sometimes downranked to control the duration, for example, when a player is mind controlled. Rank 1 Frostbolt is used in pvp for the snare effect since its cast time is shorter. Two different ranks of Flamestrike may be alternated to allow stacking ground DOTs. Occasionally (Anub'arak in ToC for example) healers may down rank to control the amount of healing, I suggest you simply disable the addon for this fight. If you know of other valid uses of downranked spells, please let me know.

You can turn off the auto-whisper, ignore downranks from certain players, get a report of all the downranked spells seen, and of course, disable the addon completely.
Type /rankwatch for help.