Don't bet against Tom Landry when he has extra time to prepare the Dallas Cowboys for a game.

Those words are accepted as an article of faith by many persons who play the point spread. They apply to Dallas' first game of the season and to the game following the Thanksgiving Day contest. Landry is always tough, the thinking goes; when he has the luxury of three additional days to prepare for a particular rival, he is money in the bank.

The statistics, I'm told, bear out this belief. Dallas has won 13 of its 14 games following Thanksgiving and, in so doing, has covered the spread.

Well, I am not about to argue over Landry's ability to get his squad ready for a game. The coach's overall record attests to that, and that ability certainly isn't restricted to opening week or to the first Sunday in December. But too much is made of this sort of statistic.

Monday night, for example, Don Shula was pictured by ABC-TV as being 14-1 against National Conference teams in the Dolphins' last 15 such outings. Tampa Bay won, impressively, although it certainly wasn't Shula's fault. His quarterbacks performed poorly.

No one should make a habit of betting against either Landry or Shula. It would be costly. But given the right situation, those statistics shouldn't scare you. In fact, those statistics tend to be built into the spread occasionally, making the line a point or two higher than it should be. When that happens, the Landry or Shula opponent might become a bargain.

In approaching Sunday's Dallas-at-Washington game, I couldn't care less about Landry's post-Thanksgiving Day record. Each matchup is separate and distinct. And I don't see why the Cowboys should be favored by 2 1/2. I made it even. The one ingredient absolutely necessary to beat Dallas is a strong offensive line. Give any passer some time to throw, and the Cowboys' secondary is very vulnerable. Joe Theismann should have enough time.

The Redskins, right now, are on a roll. The ball is bouncing in their direction, just as it did for Oakland and Jim Plunkett two years ago and for the San Francisco secondary in 1981. Every time you looked at TV last year a 49ers cornerback or safety was coming up with a deflection or an interception.

This is not to suggest Washington is on the way to the Super Bowl. The team is at least a year away from that, while Cincinnati is rapidly emerging as the team for this miniseason. But given 2 1/2 points, I can't resist the Redskins for an imaginary $250 Sunday. They have more than luck going for them. Against Landry, they'll need it.

I'll also take Philadelphia, against St. Louis, and Minnesota, at Miami, for $250 each this week. Wilbert Montgomery returns to action for the Eagles. He is central to Dick Vermeil's offense. There is nothing wrong with the defense, which should be able to exploit the inconsistency of the Cardinals' young quarterback Neil Lomax. The Eagles at home are favored by six.

Minnesota is probably the streakiest team in the NFL. The Vikings' defense is impossible to predict, but Tommy Kramer knows how to throw the ball into the end zone. I have a hunch the Vikings are beginning a roll, offensively. The Dolphins, meanwhile, must decide who is going to be their quarterback. David Woodley never has impressed me as much as he has Shula. Miami is favored by six.

In other games, Las Vegas lists San Francisco by 2 1/2 at Los Angeles tonight, Vince Ferragamo starting in place of the injured Bert Jones for the Rams, while Sunday finds Denver four over Atlanta, Green Bay two over Buffalo, Cincinnati 14 at Baltimore, the Giants seven over Houston, Pittsburgh six over Kansas City, New England at Chicago even, San Diego 2 1/2 at Cleveland, the Raiders seven over Seattle, New Orleans' remarkably improved Saints 2 1/2 over Tampa Bay and, Monday night, Detroit one over the Jets. I wanted to take Detroit for $250 but the Lions apparently still are on strike, or at least their passers are.