Q:

Is it wrong to use a Vue.js data property inside a method without passing it as argument? 

Imagine I have the following 2 scenarios:
Scenario 1:
data(){
    return {
        number: 2
    }
},
methods: {
    multiplyByTwo(number){
        return number*2
    }
},
mounted(){
    this.multiplyByTwo(this.number)
}

Scenario 2:
data(){
    return {
        number: 2
    }
},
methods: {
    multiplyByTwo(){
        return this.number*2
    }
},
mounted(){
    this.multiplyByTwo()
}

As you can see, in both cases, the function multiplyByTwo returns the data number property multiplied by 2, however, in the first scenario, I pass the data number property as an argument for the function, and in the second scenario, I straight up access the data number property from inside the function without passing it as an argument. 
Now I'm wondering if one of those methods is better or worse than the other.  I feel like scenario 2 is less flexible because in order for the method to be used, the component would have to have a data property number, otherwise it wouldn't work. 
Whereas the method in the first scenario would be reusable anywhere because I have to pass the number as an argument. 
Sadly even though I think that way, I've used scenario 2 quite a lot in my Vue.js application and I'm thinking about rewriting some methods if this is badly written code. 

A:

It depends on the situation. 

If the method is side effect free and may not not always operate on the component's state, then you should pass whatever argument is needed.  This would make sense if multiplyByTwo(this.number) and multiplyByTwo(4) were both equally likely. 
If the method has side effects and only operates on internal state, like your example, I think it's a judgment call.  However, choice of name will provide more clarity.  If I read doubleTheCurrentNumber, I wouldn't be surprised by a lack of parameters. 
If the method always operates on internal state then it doesn't make sense to pass state variables as an argument.  For example: insertDocument() may make an API call based on filling out a form. 
If the result of the operation needs to be reactive, but isn't called directly then you'd want a computed property.  A filtered list is a good example of a computed property.