How critical are the torque specs for the cowl and related parts?

Hard to say what "reasonably tight" means to you. How well calibrated is your wrist? If you've used a torque wrench a lot, you get a pretty good feel for what 50 inch pounds (or about 4 ft lbs), for example, feels like.

Another way is to estimate what that force is on a short ratchet. For example, 48 inch pounds is one pound applied to a 48 inch bar. This is the same as 8 pounds applied to a 6 inch ratchet. So, if you can estimate 8 pounds of pressure, you should be able to get very close to 50 inch pounds.

With these small numbers, it pays to err on the light side, but, as you say, "not too light." After a few years of working with plastic covers etc., you get the feel for it.

 

The reason that I provide torque specs is to eliminate ambiguity about what "reasonably tight" is. Imagine that you are driving at night in a pouring rain and then your windshield wiper assembly comes loose. Not pleasant. Also, if you overtighten, then you may strip some of the bolts which is highly irritating if it happens to you.

I suggest that if you are going to DIY, that you invest in a couple of decent torque wrenches, 3/8" and 1/2" size. This will give you a nice range of in.-lb and ft.-lb to handle almost all jobs except maybe suspension removal.

For example,
http://www.sears.com/shc/s/p_10153_...chets+&+Sockets&sName=All+Open+Stock+Wrenches

and

http://www.sears.com/shc/s/p_10153_...chets+&+Sockets&sName=All+Open+Stock+Wrenches

Those are the wrenches that I own.

I recommend against buying the cheapest torque wrenches that you can find (i.e., Harbor Freight) because this gives you an unwarranted sense of security. Really cheap tools can fail at inopportune times and might cause an accident resulting in personal injury (say if a socket splits while you are exerting significant force.)

There's no need to purchase professional grade tools (i.e. Snap-on) if you are not a mechanic earning your daily bread with those tools. Good luck.