Basic Question related to Negation IS INITIAL or IS NOT INITIAL

Hi Experts,

I am in confusion regarding very simple conditional statement.

which one is preferred in the following condition statements.

1. IF NOT so_vkorg-low IS INITIAL

Perform validation

ENDIF.

2. IF so_vkorg IS NOT INITIAL.

Perform validation

ENDIF.

3. IF so_vkorg-low IS INITIAL.

Do nothing

ELSE.

Perform validation

ENDIF.

I have read some where checking negation in a condition statement is not preferred, Instead go for other way around .

Expecting some valid points from experts

Thanks

Sreeni