ADDITIVES....GOOD or BAD???

How harmful are the additives that are sold to clean fuel injecters and the
like..

I have a 1994 Integra with 209,000 miles and it still purrs and runs great.
For about a year I have a cold weather  reving problem that lasts for about 1-2
minutes upon startup and when it has been sitting for a couple of hours.  It
revs from 1400 RPM to 2000RPM rapidly until it warms up and then settles down
to a "normal" idle...about 800RPM.  
My mechanic said to try a few tanks of premium gas( I have always used regular)
and I am on my second tank and upon startup it is getting better, but after
sitting for a couple hours I still get the reving problem!....In case this does
not work I was considering a "cleaning additive " of some type. 

Any thoughts???