This is inaccurate. So is the statement above about only using dry yeast for a clean profile.
If you want to know how much yeast is appropriate, check mr. malty's calculator. Over pitching above his recommendations will not hurt you whereas under pitching may provide an environment for under attenuation or stressed yeast, both leading to off flavors.
If you are using the big 11g packets, one may be sufficient for smaller and some normal gravity 5g batches. For any 10g batch you should at minimum be using two packets and more than likely two per 5g.
Under pitching is a major source of new brewers problems. It leads to all sorts of problems and is simple to remedy. Also, dry yeast in my neck of the woods is around $1 per pack.... too pricey???
I use almost exclusively liquid yeast because I feel I can tailor my beers better and get better results. Many like dry yeast, I just have not been as impressed with the results.
In closing, you should become friends with the search function. There was just a very good thread done about this and other topics.