Poker players aren't the only people who have to know when to hold 'em and know when to fold 'em. Thoroughbred trainers have no more important job than deciding when to manage stakes horses cautiously and when to be aggressive.

If a horse is pushed too hard at the wrong time, he can be ruined. But if he is babied too long he can miss opportunities and lose his competitive edge. Both these extremes, as well as the golden mean between them, are represented in Saturday's fascinating Whitney Stakes at Saratoga.

The historic $150,000 event has attracted four outstanding horses: Sunny's Halo, Linkage, Island Whirl and Deputy Minister. But the outcome of the race may be decided as much by the trainers as the horses' ability.

David Cross, who trains Sunny's Halo, admits that even he isn't happy about everything he has done with the Kentucky Derby winner. The colt's triumph at Churchill Downs was followed by speculation that he would be syndicated for $10 million or $15 million. Since then, the syndication possibilities and the owner's wishes have often dictated Cross's actions. "We haven't exactly been doing what we wanted," he said. "There's been no joy at all since the Derby."

Cross said he had misgivings about running the colt in the Preakness; after his defeat there, he made an ill-conceived trip to Arlington Park and lost badly there, too. Now Cross is aiming Sunny's Halo for the Travers Stakes, and Sunday's Jim Dandy Stakes would be the logical spot to prep for it.

But Cross is entering the 3-year-old against tough older horses Saturday because a defeat in the Whitney will hurt his reputation less. "I've devalued the horse enough as it is," the trainer said glumly. "At least, if he gets beat by older horses he won't lose any more prestige."

If Cross has erred by putting Sunny's Halo in races in which he shouldn't have run, Henry Clark has gone to the opposite extreme with Linkage. Training for a very rich owner, Clark has never had to worry about winning purse money with his well-bred horses, but with Linkage he has taken his normal conservatism to bizarre lengths.

Last year, the colt would have been the standout favorite to win the Kentucky Derby, but Clark wouldn't run; Linkage didn't win again all year. This year, Linkage has been in steady training for months, but he has run only in three easy allowance-class sprints. Now he is facing some tough battle-tested rivals in the 1 1/8-mile Whitney, and they may well be sharper than he is.

A good trainer has to find the right blend of patience and aggressiveness, and nobody has combined these qualities any better than Reynaldo Nobles, the trainer of Deputy Minister.

The colt was the 2-year-old champion of 1981, but his reputation had hit bottom when Nobles took over his training this winter. Deputy Minister began a successful comeback, but just as he did he suffered a foot infection.

"We had to cut away nearly all his foot to clean out the pus pocket," Nobles said. "Then we gave him all the time he needed to regrow the foot."

Deputy Minister was sidelined for months, but he trained so splendidly for his return to competition that Nobles outlined an aggressive program for him. The colt ran in the Tom Fool Stakes at Belmont and blew away a field of top sprinters. After the Whitney, Nobles will continue on a course that he hopes will bring Deputy Minister the horse-of-the-year title.

His plans could very well be thwarted by Hall of Fame trainer Laz Barrera and Island Whirl. The speedster won the $500,000 Hollywood Gold Cup last month, and Barrera has brought him here in razor-sharp condition.

The speed-favoring Saratoga racing strip should suit him perfectly, just as it will hinder the stretch-running Deputy Minister. If either of them wins, the outcome will verify the importance of good training. If Sunny's Halo or Linkage wins, the result will demonstrate that there is no justice in the world.