What's the proper way to use ctags?

[Phoenix Yuan]

Hi,

Let say I want to quickly browse through all the source files in a given 
directory. What I tried was I ran ctags -R source_dir, which builds a 
tag file for the entire source. When I start vim in that directory and 
view files in the sub directories, tag works just fine. But when I go 
into other directory(outside of vim), since there is no tag file there, 
I can't use the tag feature anymore.

I could ran ctag in all sub directories, but then I won't be able to 
search for functions that are define outside of the sub directory. So 
what's the proper way to use ctag?

PS: anybody know how to redefine the ctag path for vim? I downloaded the 
latest from source forge but my admin said I can't replace the one in 
/usr/bin...

thanks!