Whether your dog exhibits reactive or aggressive behaviors, or you just have a dog in the early stages of basic training, the key to success is early intervention, before unwanted behaviors start.
Take jumping, for example. Some people wait until the dog jumps up, then say "Off!" (or some variation). But, dogs don't learn via our words, they learn through consequences.
Jumping is generally an attention-seeking behavior. If the dog jumps on a visitor, and the visitor looks at, touches, or talks to the dog, any intervention by the dog's owner will be too late. The behavior has happened and been reinforced.
Modern training dictates that we don't wait until a dog jumps on someone. We want to anticipate that the dog is likely to jump (because of their age, the stage of training they're in, or their past behavior) and take steps to prevent the jumping. What those steps are depend on the scenario.
It might mean the dog stays behind a baby gate while guests enter, until the initial excitement dies down, then they can come in on a leash. Or, it might even mean telling a friendly stranger that the dog is learning not to jump and either a) giving the person instructions on how to assist in that training, or b) wishing them a nice day and continuing the training from a distance.
Even if the dog wasn't going to jump, it doesn't matter. You were ready, just in case. And, hopefully, you were reinforcing the heck out of the "good" behavior.
While it speeds the training process for basic manners, it is absolutely imperative when it comes to working with fear, reactivity, or aggression.
When I'm working with a dog that has a history of reactive behavior, I am just going to assume that something new in the environment is going to trigger a reaction. If the dog is reactive to dogs, I'm still going to use a passing cyclist for practice. Or a jogger. Or a guy with a leaf blower. Anything new or different is an opportunity to practice.
I start training the moment that thing appears. Not when I see the dog's body language change. Not when the dog starts to growl. Not when the barking begins. While the dog is still as calm and attentive as they were before the environment changed.
Even if the dog wasn't going to react, we had an easy practice session. It is never wrong to reinforce "good" behavior as often as possible.
Waiting too long before intervening is one of the biggest mistakes people make in training and behavior modification...and one that is very easily remedied.
Intervene early. Reinforce generously.