How to keep a high resolution when rasterize

"I was wondering if I still need to rasterize my line work when I'm working on an exact size of paper that I want print or show."

You lost me.  Why all the trouble?  Why wouldn't you be able to "print or show" at "exact size"?  Your workflow sounds off.

"So I have my drawings on letter paper and whenever I rasterize the line work and export it as png the resolution is still too bad."

The problem is the highest resolution is 300ppi.  For line work, you need a minimum of 1200ppi @ 100%.  All the more reason to keep the artwork vector.  A possible workaround would be to enlarge a copy of the original artwork to 35"w x 44" h > Export as .tiff @ 300ppi ( grayscale? ).  Then, in Photoshop, reduce back to 8.5w x 11" h @ 1200ppi.

"It would be nice if I can know what's the best way to save or export the line work to work with photoshop later..."

Could you explain why you have to go to Photoshop?  Why rasterize?  Something's missing.

if you're going from AI to PS, it'd be better to not rasterise in AI at all, but place your work into PS as a smart object. then you only need to rasterise it once you know the dimensions you'll be working at.