It's ok to take a sample whenever you want to, or need to, as long as the utensil is sanitized before sticking it in your beer.
I usually don't take an SG until at I'm planning on doing something with that reading. What I mean is, if I'm going to let it sit for another week, no matter what the reading, then I don't bother taking a measurement. If I want to check the SG to see if it's ready to bottle in a few days, or ready to rack, then I go ahead and take a sample. You want harm the beer by opening it and taking a reading.