I was re-installing Windows for a customer that fell for a phone tech support scam (the scammer had control for quite a while). I was unable to run antivirus scans (on multiple programs, including Malwarebytes Chameleon, it said unable to scan after starting), but had no other visible solutions. After seeing the customer had little personal files or programs installed, I decided a re-install of Windows was the way to go. I started a drive wipe out of an overabundance of caution, but after seeing it would take 12 hours, decided it was not worth the time as well as the wear and tear on the drive. I re-installed Windows and gave the computer back to the customer (and scanned it both with Malwarebytes and Panda to ensure no viruses had been transferred), but have been worried of a boot sector virus that may have remained after a reformat. Is this a valid concern?
If they haven't came back to you with further issues as of yet, Id say they got lucky... Compared to what I had to go through with fixing a friends laptop that had a malicious installer that fcked everything (linky http/www.tomshardware.com/answers/id-2409870/sisters-friend-weird-virus.html)
First off, I'd only recommend formatting as a last ditch effort to clean your PC. Since most viruses can be removed relatively easily, and few of them really root themselves deep, it's best to just do it by hand; it also gives you a much better look at what is currently infecting computers.
On the other hand, a reformat is typically enough to prevent the customer from having any more issues. Doing a drive wipe is always worth it, if you genuinely think the virus may survive the reformatting and re-partitioning. While the virus shouldn't be able to remain persistent, why not wipe it any way? Wiping is a much more secure form of cleaning the drive, so you might as well do it.
If the virus did manage to infect a boot sector, a full wipe is in order, and a format (depending on how it's done) may not be enough to wipe it out. If there is no complaint from the customer yet, then the reformatting most likely did the job. You might want to contact the customer, though, and ask to see the computer (free of charge, of course) to tell them you'd like to double check your work. While some people might feel a bit worried, reassurance that you're simply trying to make sure they are safe, and that you completed the job is something that is kind of rare, yet quite nice. Depending on the customer, they might be quite taken by the suggestion, and find it more of a positive move on your part. It's an option to consider.