The short answer is yes, you should always wear a respirator while working with wood. 

The long answer is it depends on exactly what you are doing and how much you value your respiratory system.  Our respiratory system does a pretty good job of filtering out larger particles such as wood chips.  It's a completely different story when it comes to fine particles barely visible to the naked eye or even not visible at all.  Over time, these fine particles can cause various health problems ranging from allergies all the way to cancer. 

You might work with wood a lot in a closed garage with no filtration at all and think to yourself, "This isn't affecting me at all, I should be good."  In reality, you might be completely fine and it might not affect you at all, but why take that chance?  There are many instances where people have had to retire early from woodworking because they became allergic to the wood particles.  

Now some of you might be thinking, "But I have a dust collection system, I should be fine."  While a dust collection system is definitely a step in the right direction, most collection systems only filter down to 5 microns with fewer filtering out 1 micron or more.  It is possible to get a HEPA filter for your dust collection system that would filter out particles all the way down to .3 microns if you purchased a True HEPA filter.  "Ok great, I'll just get a dust collection system with a True HEPA filter in it so I don't have to wear a respirator."  If you feel that this system is collecting 100% of the particles in the air then there is no need for a respirator, but what about those particles that just barely make it around your collection hose and continue to float around you until eventually collected? 

Some of you might also be thinking "I work outside, so there is no need for a respirator."  While this scenario is much better than working in a confined space, it is still not foolproof.  What happens when the wind is blowing in your direction instead of away from you or worse yet when the wind isn't blowing at all and all of those fine particles just sit in the area surrounding you with nowhere to go?  You might not think they are there, but the particles that are not visible to the naked eye are the most hazardous.

Now you're done sanding/cutting/planing and are ready to clean everything up for the night and finally ready to take that respirator off.  Hold on to that thought because not only can dust particles hang around in the air for quite some time, but it's also the smallest, most dangerous particles that hang around the longest.  So even if you just stand there, not moving at all, it's still potentially dangerous to be in that area without a respirator on.  Unfortunately, cleaning up for the night involves moving things around and removing all of the sawdust mess you just made which in turn will stir up even more dust for you to breathe in.  

Respiratory health is all in the eyes of the beholder, there is no law requiring you to protect your own lungs and overall health.  If you feel comfortable that your system is "good enough,"  then, by all means, move forward with that plan.  I, however, will be wearing a respirator any time I start woodworking up until the time I walk inside my house at the end of the day.