A limiter usually changes the sound if you have to raise the level of the mix more than a few dBs.  Since most of us mix to -6 to -3dB, the mix will usually need mastering.  And if you don't know how a limiter effects a mix, it is hard to mix w/ that in mind.  As most things engineering, it takes experience.  And different styles of music need different amounts of mastering.  A hot rock and roll may need a kiss of limiter, while the ballad on the same song needs more to make it sound right, but the mastering makes it sound ... strained.
 
So I like to keep the 2 processes separate.  I do the mix, print it at the project rate, and then use SF and Voxengo to do a rough master.  I can put that on CD or just import the "master" back into the project and solo/mute it to see how it sounds.  I can then use that to change the mix if necessary - such as remixing the above ballad hotter so the mastering doesn't sound so strained on it (I don't need to say you must have no effects on the master buss for this to work).  It is a good compromise.  You get to learn about mastering, even if you don't do it yourself.  And you learn how the limiting of your music effects the mix so you become a better mixer.
 
Or you can just enable/disable your master buss limiter to achieve the same thing.
 
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One more question.. why do people master in soundforge or wavelab?

I've been told that Cd architect is pretty handy.. but Sonar is capable of mastering?

You can use a range of programs to master in.  I actually use Harrison Mixbus for a lot of mastering and there are various reasons for it.
 
It has a beautiful sound and just makes your mixes sound better for being inside it.
The EQ is very cool and is different on Tracks, buses and the main stereo buss.
The Harrison mastering plugins are also excellent.
 
You can certainly do it in Sonar and Studio One is also cool for mastering in that it offers a special mastering page.
 
I still believe that a carefully controlled mix will not be effected at all much with limiters and compressors over the mix.  The key is using VU meters during the mix process and understanding the ballistics of the VU meters and knowing when things are good and not so good.  I bet if a mix changes a lot under the limiter or compressor when you put a VU meter over that mix you will see why just by the movement of the meter and how it dances to the music.  Great mixes just make the VU move in a certain way.  And compression or limiting does not really change that too much.  Although it is also very interesting what happens to the VU meter ballisitic the moment you do put a compressor over it.  When the compressor is not well adjusted the VU meter ballisitic changes a lot. (in a bad way)  When it is set right the VU goes back to dancing correctly like it did before the compression went on. (in the top part of the meter range that is. What happens is it should just not drop down so quiet that is all)
 
You can almost mix with a VU meter and you can certainly adjust a compressor very well with one too.
 

Elffin
One more question.. why do people master in soundforge or wavelab?

I've been told that Cd architect is pretty handy.. but Sonar is capable of mastering?

wavelab was designed specifically for mastering.
 
again, it's just a tool.
but it's a damn good one.
that's what i use
 

SONAR is quite capable of "mastering." Stereo editors are better since that is one of the main jobs they are designed for.  They are a better tool for a job.
 
SF is usually faster to work on stereo files. DAWs will save a project.  SF saves a sound file w/ its attached history.  A clip in SF is usually bigger than a track in a DAW - yea, I know you can save a template or screen set but SF comes up ready.  Maybe I'm just used to SF, but top and tail, splicing etc. are all easier than in SONAR.  It is easier to zoom in on whatever needs editing.  My computer is quicker to perform bounces than going through a DAW.  Finally, you can save CD and MP3 info, as well as notes etc. w/in the file.  Which means CDA is ready to go when you pull a file into the time line. 
 
Most of the difference is in time and ease and SF is very intuitive.  You don't have to have it, but it helps.
 
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