This is one of those things that is a grey area and a bit of a personal preference. Generally speaking, it is safe, yes. However this command performs the same validation function that
request system software add /var/tmp/image.tgz performs. In the most general sense, the validation component initializes instances of the daemons from the new version of code and checks current configuration, etc. against them, however they won't perform any major functions (i.e. routing).
You will consume some additional resources on the device so be conscious of your hardware limitations or current load on the router. Again though, unless you're on something older like an RE2000 or really pushing the limits of your router, you should be fine.
That said, I've never seen it cause a problem.
Bear in mind that there are times where the validation will fail, period. Since we're talking about MX (not including MX5 variants), a specific example is when you are on something pre-15.1 and you're going to something that's 15.1 or later. The FreeBSD kernel version pre-15.1 is 6, 15.1 and later it's 10 or higher depending on the specific target version. Regardless, when validation tries to spin up daemons meant for a newer kernel (10+) on something running version 6, it fails because they're just incapable of initializing without the upgraded kernel. This is why in some cases JTAC actually recommends you skip validation by using the
no-validate flag on the upgrade.