Hmm. "exact rounding" isn't (since the purpose of rounding is to NOT use the exact value).

Just be careful with the "rule of thumb". You want to actually be aware of what the data is that you're representing, and how you plan to use it. Exact data type might be awesome for final results, but if what you're looking at is already an approximate value or is logically an "intermediate value", it may not be accurate to treat every digit as significant. In some of those cases - the "exact type" will be rounding/truncating intermediate steps when it isn't appropriate to do so.