The NFL Combine is a time for teams to do their due diligence.

They watch, they interview and they study. As San Francisco 49ers General Manager Trent Baalke said last week, a team needs to do its homework so it can be prepared to make “decisions on the fly” when opportunities arise.

The common perception is that the 49ers, with the 30th overall pick in the first round of April’s draft, will take a wide receiver to upgrade a position that was woefully thin and unproductive by the end of last season.

But that isn’t necessarily the case.

If the Niners decide to jump into the free-agent market to sign a proven talent such as Marques Colston, Vincent Jackson or Dwayne Bowe, that first-round pick could be used on another talent. Or, the 49ers could pass on a first-round wideout and find what they need in later rounds because Baalke says this draft is deep in receivers.

“Some of those guys at the bottom end are going to end up being good players,” Baalke told the San Francisco Chronicle’s Eric Branch this past weekend at the Combine in Indianapolis. “You got to find out which of that group at the bottom of those 15 or 20 guys, which three or four are really going to springboard up and improve as a professional? That’s why we’re all here, to try to find that nugget that’s going to lead us to one of those guys.”

The top wide receivers available in the draft include Justin Blackmon of Oklahoma State, Kendall Wright of Baylor, Michael Floyd of Notre Dame, Mohamed Sanu of Rutgers, Rueben Randle of LSU, Alshon Jeffrey of South Carolina, Joe Adams of Arkansas, Arizona’s Juron Criner and Stephen Hill of Georgia Tech.

On Saturday, Branch reported Sanu already had met with the 49ers at the Combine, and Randle was scheduled to meet with the team.

While the top-rated wide receivers drew plenty of interest Sunday when they ran and tested at the Combine, ESPN’s Mike Sando noted today that many of those receivers also have perceived flaws. So a team in need of impact players at the position – such as the 49ers – should not necessarily fixate on taking for need in the first round. A team could end up getting a player who might not be the answer to its questions.

Todd McShay of Scouts Inc. told Sando that he isn’t sold on the highest-rated receivers this year.

“I think they’re all overrated,” McShay told Sando. “That doesn’t mean they’re not going to be good. I don’t think Justin Blackmon is A.J. Green (now with the Bengals) or even Julio (Jones, with the Falcons) or even Michael Crabtree (of the 49ers). He’s really, really good, but certainly not Calvin Johnson (Lions) or A.J. Green.”

McShay also added that he believes Baylor’s Wright drops “a lot of passes,” has doubts if South Carolina’s Jeffery can “separate” from DBs, thinks Sanu is “way overrated” and that LSU’s Randle “quit” in the national championship game.

What does it all mean?

That the 49ers do, indeed, have a need to upgrade at wide receiver. But with several weeks to go before the draft and unrestricted free agents becoming available on March 13, the Niners are still deep in due-diligence mode.

Baalke, who says he’s “methodical” about sticking to his plan this offseason, isn’t about to tip his hand yet.

Especially when history shows some of the NFL’s most productive wide receivers last season – “nuggets” such as Wes Welker, Victor Cruz, Brandon Marshall, Colston and Stevie Johnson – were undrafted free agents or taken in the fourth round, or lower.