Steinerman maintains an "overweight" recommendation on Menlo Park, California-based Robert Half, which is trading at a multiple of 1.06 times enterprise value to sales for the last 12 months. That is close to the 2001 recessionary trough and a "pretty substantial discount," he said.

Signs of "incremental improvement" in the U.S. labor market "aren't being accurately priced into the sector," said Jonathan Upton, investment strategist at Lamkin Wealth Management. The Louisville, Kentucky-based investment company is "bullish" about staffing, even as industries such as technology and financial services have "garnered more attention from investors recently," he said.

The Bloomberg staffing index has yet to break through a so- called double-top -- a relative high set in October and January -- because "investors lack conviction in this sector right now," said Jim Stellakis, founder and director of research at New York-based research company Technical Alpha. If the index surpasses that relative peak, it may see an "accelerated move higher," he said.

Employment-Outlook Decline

One sign of uncertainty is a decline in the employment outlook for the next six months. The share of Americans who say there will be "more" jobs minus the share who say there will be "fewer" fell to minus 1 percentage point in March, after the first positive reading in a year for February, according to data from the Conference Board, a New York research group.

The so-called temp-penetration rate -- the portion of temporary workers relative to all employees on nonfarm payrolls -- is more positive. It rose in February to 1.86 percent, the highest since November 2007, the month before the recession began, Steinerman said, citing Bureau of Labor Statistics data. The rate is a "leading read" on labor demand and a "bullish concurrent indicator" of the economy, he said.

Shares of staffing companies with domestic exposure are generally "doing better" than those with more business in Europe, said Tobey Sommer, an analyst in Nashville at SunTrust Robinson Humphrey Inc.

'Good Bet'

On Assignment Inc., which generates about 95 percent of revenue in the U.S. and specializes in technology, is attractive because it's exposed to the U.S. economy and segment investors "want to be in," said Sommer, who has a "buy" recommendation on the Calabasas, California-based company.He has the same recommendation on Insperity, based in Kingwood, Texas, which may be a "good bet on small-business job growth," he said.

Small-business sentiment is rising, which Sommer said is encouraging because these companies make up the largest portion of the labor market. The National Federation of Independent Business Optimism Index rose to 94.3 in February from 93.9 in January, the sixth straight month of gains and second-highest reading since December 2007, association data show.

This optimism was echoed at an annual staffing forum in mid-March, where attendees said they were confident demand would "remain solid this year," Timothy McHugh, an analyst in Chicago at William Blair & Co. wrote in a March 16 report. Companies at the conference said January and February were "strong months," and some attendees said that weekly revenue trends have ramped up recently, he wrote.

Conversion Rate

As companies begin to report earnings next month, Camden said investors should pay attention to the so-called conversion rate: temporary employees hired on a permanent basis. "Staffing firms are seeing an increase in temp-to-perm conversion that is typical at a second stage of a recovery," he said.