Question: What should be the preferred way by programmers:

1) Only Write:

```
SomeBoolean = True

```

2) Read but write only if necessary

```
If Not SomeBoolean Then SomeBoolean = True

```

Answer: 
It's really hard to know the answer to this without knowing more about the environment. It seems a reasonable check would be to run some performance tests by iterating over this task many, many times.

Empirical evidence sometimes is surprising compared to what you'd expect.
Assuming you are referring to a RUNTIME context and a shared variable:

In a multiprocessor environment, unnecessary writes can lead to performance degradation: cache flush, synchronization overhead etc.

So YES it can make a difference... get profiling **if** the situation lends itself to it.