I have a complicated graphic in Illustrator that will be used multiple times in files. It's making the file very heavy to work on smoothly. Can I make it into a graphic symbol for offset printing? Will it effect the details / resolution in offset print later. Have never used it before so please help.
Making it a "graphic symbol" has nothing to do with off-set printing. The general document color settings are what effect printing and if you decided to import that into something like InDesign then you'll have to check the color/press settings in that app as well.
It might help with lightening the file size by creating multiple instances out of one symbol.
If your original artwork is the proper quality for offset printing, all symbol instances will be the same quality.
Symbols simply reference the original artwork rather than copying it. Symbols do not change the original artwork, unless you use some Symbol tool -- like the Symbol Stainer on it.
Basically, All symbols are the same as if you copy/pasted the original art. The only difference is how *Illustrator handles the data**. The data never actually changes though.
So YES -- using symbols is fine for offset printing.