As others have opined, yes and no. Check this
to bear me out.
The rule of thumb is a certain number of active
yeast cells per point of gravity per milliliter of wort. There are many reasons for this, as there are a number of flaws yeast can cause in the finished beer based on over- or under-pitching. Attention must be paid to how long the yeast has been sitting around - an old culture will have severely compromised viability. Also, attention must be paid to how the culture has been stored - temperature extremes are as bad as age. Call me a pessimist, but I always assume my yeast has been abused, just in case it has been!
Let's get practical.
A 1.050 wort requires ~185 billion active yeast cells for proper inoculation.
An 11g packet of dry yeast contains more than that number, so that doesn't need a starter; just rehydrate according to the instructions on the packet and pitch. Dry yeast is nowhere near as susceptible to temperature extremes as liquid cultures, and can be stored longer before viability becomes an issue.
A vial of White Labs yeast contains, optimally, 140 billion cells. Unfortunately, that number is no longer reliable once the yeast is packed for shipment. Once it's shipped to the homebrew store, it's been shipped once - potentially exposing it to temperature extremes. Check the manufacture date on the package carefully. Significantly, the numbers, even when the culture is new
, still require a starter for proper pitching. Thus, it's wise to always
make a starter for beers over OG 1.037 when using White Labs yeasts, or buy two vials.
The "Activator" packs contain, according to Wyeast, "a minimum of 100 billion cells". What they really
say is that each package contains 1.2 x 109 cells/ml. (That's 1.2 billion, by the by). No matter how you slice it, even the Activator pack is insufficient to inoculate a wort stronger than 1.026. The Propogator pack is even worse, with ~25 billion cells in a pristine pack. Thus, it's always
wise to make a starter with Wyeast packages also or double-pitch.
Look - I'm not trying to scare you. I'm just giving you the numbers. Of course
the manufacturers' websites are going to tell you what you read; do you really think they'd do otherwise?
But the numbers are the numbers.
Keep in mind that many brewers experience excellent results from simply using the packages of liquid yeast, regardless of what the numbers say. They pitch the packet and RDWHAHB. Maybe that'll work for you. I'm a little too OCD for that!
If you don't want or are ill-equipped to make starters, don't despair! Just keep pitching as best you can. Oh, and make good beer!