Checking if a method returns false: assign result to temporary variable, or put method invocation directly in conditional?

Question

Is it a good practice to call a method that returns true or false values in an if statement?

Something like this:

private void VerifyAccount()
{
    if (!ValidateCredentials(txtUser.Text, txtPassword.Text))
    {
        MessageBox.Show("Invalid user name or password");
    }
}

private bool ValidateCredentials(string userName, string password)
{
    string existingPassword = GetUserPassword(userName);
    if (existingPassword == null)
        return false;

    var hasher = new Hasher { SaltSize = 16 };
    bool passwordsMatch = hasher.CompareStringToHash(password, existingPassword);

    return passwordsMatch;
}

or is it better to store them in a variable then compare them using if else values like this

bool validate = ValidateCredentials(txtUser.Text, txtPassword.Text);
if(validate == false){
    //Do something
}

I am not only referring to .NET, I am referring to the question in all programming languages it just so happens that I used .NET as an example