Is it advisable to crack the lid on the fermenter and take a reading with a sanitized hydrometer or should I continue to use the spigot? Seems like a waste of good beer to keep dumping it down the drain when I’m done. Or can I use a sanitized hydrometer tube and pour it back in the fermenter when I’m done? Thanks!
You need such a small amount of beer to take a hydrometer reading that I don’t think I’d take the (probably) small risk of pouring it back in. I open my fermenter lid and use a sanitized turkey baster to take a sample for readings. I’ve never trusted the spigots on fermenters…it seems all to easy for an infection to live in there.
Most folks drink theirs, for educational reasons. Over time you develop a sense for what the final cold-and-carbonated beer will taste like.
But do be careful, there may be alcohol in the sample. 
Indeed. I had to do that very thing this morning at 9:00 to check my semi-open dunkel fermentation, which is sloooowwww. Fun way to start the day, buzzed on coffee and yeasty dunkel.
If the fermentation has a long way to go, I’ll dump the sample into my wort collection jar (for starters) in the freezer.
And yes, Denny is very right about possible infections in fermentor spigots. If it’s plastic, getting inside to clean it out is next to impossible and I don’t trust on “star san creep” to be an effective sanitation mechanism if it can’t be physically cleaned, first. This problem ruined several otherwise excellent beers I had made, until I finally found the problem. Yes, I’m still bitter about it.
For that reason, I put removable ball valves on my plastic fermenters. Much easier to have peace of mind about sanitizing those!