Taking lid of during fermentation

Is it ok to take the lid off of my fermentation bucket during the fermentation process in order to get a gravity reading?

Yes.

It's about minimising risk no irradicating it.
Just take sensible precautions to prevent things dropping in it and you'll be fine.
Also make sure what ever comes into contact with the beer is sanitised.

I usually just spray the lid and/or the carboy stopper with sanitizer (a sray bottle with star san or iodophor in distilled water lasts a looooong time and is great for sanitize the lid and your sample puller) before I open it, and set it aside to draw my sample. Then I quickly respray it before replacing...You can do it in less that 30 seconds, even less if you have a friend help you...they can lit the lid, you draw the sample and they spray and close it back up. Bing...Bam...Boom.

Remember there is a cushion of CO2 sitting on the beer, that will protect it from anything while it is open.

Just keep in mind that you don't need to test it during fermentation, only when you think that fermentation may be over. I don't bother to test before 10 days, and only then if I need to know where it's at with regard to taking some kind of action like racking to secondary. Otherwise you're better off leaving it alone until you're ready to do something with it.