In my experience, tests of the form if (ptr) or if (!ptr) are preferred. They do not depend on the definition of the symbol NULL. They do not expose the opportunity for the accidental assignment. And they are clear and succinct.

Edit: As SoapBox points out in a comment, they are compatible with C++ classes such as unique_ptr, shared_ptr, auto_ptr that are objects that act as pointers and which provide a conversion to bool to enable exactly this idiom. For these objects, an explicit comparison to NULL would have to invoke a conversion to pointer which may have other semantic side effects or be more expensive than the simple existence check that the bool conversion implies.

I have a preference for code that says what it means without unneeded text. if (ptr != NULL) has the same meaning as if (ptr) but at the cost of redundant specificity. The next logical thing is to write if ((ptr != NULL) == TRUE) and that way lies madness. The C language is clear that a boolean tested by if, while or the like has a specific meaning of non-zero value is true and zero is false. Redundancy does not make it clearer.

In C and C++, pointers are inherently unsafe, that is, when you dereference a pointer, it is your own responsibility to make sure it points somewhere valid; this is part of what "manual memory management" is about (as opposed to the automatic memory management schemes implemented in languages like Java, PHP, or the .NET runtime, which won't allow you to create invalid references without considerable effort).

A common solution that catches many errors is to set all pointers that don't point to anything as NULL (or, in correct C++, 0), and checking for that before accessing the pointer. Specifically, it is common practice to initialize all pointers to NULL (unless you already have something to point them at when you declare them), and set them to NULL when you delete or free() them (unless they go out of scope immediately after that). Example (in C, but also valid C++):

void fill_foo(int* foo) {
    *foo = 23; // this will crash and burn if foo is NULL
}

A better version:

void fill_foo(int* foo) {
    if (!foo) { // this is the NULL check
        printf("This is wrong\n");
        return;
    }
    *foo = 23;
}

Without the null check, passing a NULL pointer into this function will cause a segfault, and there is nothing you can do - the OS will simply kill your process and maybe core-dump or pop up a crash report dialog. With the null check in place, you can perform proper error handling and recover gracefully - correct the problem yourself, abort the current operation, write a log entry, notify the user, whatever is appropriate.

Invalid null pointers can either be caused by programmer error or by runtime error. Runtime errors are something a programmer can't fix, like a malloc failing due to low memory or the network dropping a packet or the user entering something stupid. Programmer errors are caused by a programmer using the function incorrectly.

The general rule of thumb I've seen is that runtime errors should always be checked, but programmer errors don't have to be checked every time. Let's say some idiot programmer directly called graph_get_current_column_color(0). It will segfault the first time it's called, but once you fix it, the fix is compiled in permanently. No need to check every single time it's run.

Sometimes, especially in third party libraries, you'll see an assert to check for the programmer errors instead of an if statement. That allows you to compile in the checks during development, and leave them out in production code. I've also occasionally seen gratuitous checks where the source of the potential programmer error is far removed from the symptom.

Obviously, you can always find someone more pedantic, but most C programmers I know favor less cluttered code over code that is marginally safer. And "safer" is a subjective term. A blatant segfault during development is preferable to a subtle corruption error in the field.