This answer is based upon my 20 years experience of having legal departments review testimonials and approve/refuse edits. I am not a lawyer though. Almost every piece I design has many, many testimonials and they almost always go through that company's legal department for review. So, while I'm not a lawyer myself, I've gotten familiar with what I can and can't do with respect to testimonials.
For testimonials, generally no, you should not edit when possible.
Testimonials are unlike other text. For legal reasons, what is reproduced must very closely resemble what was actually submitted.
You may also want to consider the fact that some grammatical errors and non-standard usage may actually lend to the credibility of the testimonials themselves. People, in general, don't write casually using absolute proper English. So, it's not absolutely necessary to stick to proper English when reproducing casual conversation.
- You can swap unclear pronouns for proper nouns at times if the testimonial is unclear what the subject refers to. Customarily these changes are denoted by brackets within the text, a la...
"It has been great!" ----> "[Company] has been great!"
"He taught me alot!" ----> "[Joe] taught me alot" (yes grammatical error was left as is)
- You can insert words if they are missing, again brackets are used to designate the insertion, a la....
"The savings have unbelievable!" ----> "The savings have [been] unbelievable!"
- Misspellings can generally be corrected without the need to designate the change.
"The savigns have been unbeleivable!" --> "The savings have been unbelievable!"
- Most grammatical errors should be left as is. Correcting a grammatical error is often more subjective based upon regional dialect and education level and can change the intent of the testimonial in some instances. Because of this, it is often best to leave grammatical errors as is.
- You can truncate a testimonial, or only use part of what was submitted.
"My cat, Snowball, hates winter. She likes to play in the leave in Autumn though. This product was fantastic at reducing her hairballs!" --> "This product was fantastic at reducing hairballs!"
Removing innocuous, unimportant, unrelated bits is generally okay. but anything which directly refers to the company/product/service should generally be left as is.
You should be aware that some legal departments have had issues with overly "cherry-picking" testimonials. Don't eliminate actual testimonial text that may be less than "wonderful" for the product/service if it's then followed by something you do wish to include. The intent of the testimonial should often remain. Again, this can also lend to the credibility of the testimonial itself.
If you are overly concerned with an unintelligible testimonial, it's best to either not use that testimonial, or have a legal department review the changes you wish to make.
Testimonials are quotes and putting words into someone else's mouth can be haphazard and problematic. You should strive to leave things as they are quoted as much as possible. It is not unheard of for a someone to see their name being used as a testimonial and be unhappy with how the intent of their quote has been changed.
"I signed up for SubscriptionB last year. I hate this company! All they do is steal from me. I wish I knew they existed years ago. Then I'd have a benchmark to avoid!"
could be edited down to
"I signed up for Subscription B last year. I wish I knew [Company] existed years ago."
Which completely changes the intent of the quote... and can get you/the company into a lot of trouble.
In my world, for my clients, the clients have to retain the original testimonial letter/email/video/whatever for comparison. And I have worked with companies in the past where the Federal Trade Commission (FTC) reviewed testimonials. So, when comparing the original quote to what was reproduced, it should always be clear that they are essentially the same quote and not overly edited to meet a sales goal.