It isn't as true for isn't, I don't think.
If I was writing for a formal audience, there are some contractions I'd avoid, and others I'd have no problem using.
Some examples where I would NOT use the contraction:
The experiment could've been set up differently. (use could have instead)
This experiment's the first one of three that we ran. (use experiment is instead)
I'd surmised the experiment would have yielded different results. (use I had instead)
This'll be the last experiment we run at this facility. (use this will)
But I probably wouldn't have a problem with using these:
This experiment doesn't invalidate previously collected data.
These experiments won't be the last of our work.
We're confident our data is free from instrumentation errors.
I don't know if there's a hard-and-fast rule for when to avoid certain contractions, but it wouldn't hurt to follow these general guidelines:
- avoid using contractions that aren't listed in some dictionaries (for example, my Mac's built-in dictionary has entries for isn't, wasn't, and I'm, but it doesn't have could've or this'll).
- avoid using a contraction that could be expanded in more than one way, if the intended expansion isn't abundantly clear (for example, he'd can be a contraction of he had or he would).
- don't use 's as a contraction for is on words like experiment or example (this parenthetical example's breaking that rule, for example).
- avoid 'd as a contraction for ed on a verb (I address'd that because you inquired about it in your question).
This website has some good comments on the matter:
Some people are under the impression that contractions should never appear in writing, but this belief is mistaken. The use of contractions is directly related to tone. In informal writing (from text messages and blogs to memos and personal essays), we often rely on contractions to maintain a colloquial tone. In more formal writing assignments (such as academic reports or term papers), avoiding contractions is a way of establishing a more serious tone.
Before deciding whether to use contractions in a writing assignment, consider your audience and your purpose for writing.
I pretty much agree with that, although, I think that avoiding contractions as "a way of establishing a more serious tone" can be taken a bit too far, particularly with the to be and not contractions, such as isn't, wasn't, don't, and doesn't. Don't be overly apprehensive about those.