IMO,
Simply holding/ weilding two weapons out does not incur the TWF penalties.
However, if you choose to take an additional attack in that round, you must incur the normal penalties that apply to all those attacks.
If you soleley hold/ weild two weapons, but only chose to take the number of attacks you would normally have according to your Base Attack, then you do not incur any penalties.
My word of caution would be, fight the same way all the time so your DM is not guessing what your doing.
Simply holding/ weilding two weapons out does not incur the TWF penalties.
However, if you choose to take an additional attack in that round, you must incur the normal penalties that apply to all those attacks.
If you soleley hold/ weild two weapons, but only chose to take the number of attacks you would normally have according to your Base Attack, then you do not incur any penalties.
My word of caution would be, fight the same way all the time so your DM is not guessing what your doing.