Hi - Don't worry about conflicting information, just use your common sense. Anywhere there are mosquitos, there is also the possibility of Malaria (and Dengue etc). The risk increases with the increase in mosquitos and economic depression and decrease in health facilities....all these things probably describe palawan to a greater or lesser degree (and indeed anywhere interesting and off-beat that one wishes to travel to these days)
The best first line of defense is always prevention, and in this case it is insect repellant. If you feel you should, then also take the Malaria pills, especially if (as you say) you are going trekking into the hills. Malaria these days is becoming increasingly resistant to medication, so the insect repellant is probably more important - but remember to re-apply after swimming, washing etc. If you are going to be sleeping in non-sealed units when you are trekking (by this I mean places that have open windows, unsealed door joints and no A/C) then it is a must to have a well arranged mosquito net - and once set up, spray the netting with repellant, which gives you excellent protection. If you are staying in proper hotels at night, with A/C and sealed rooms, then this latter should be unnecessary while sleeping.
When using repellant, be conscientious in its' use - it is easy to forget - spray also the outside of your shoes, socks collar of clothing at the back. One non-chemical defense is a floor-fan directed onto your sitting or sleeping position, as mosquitos don't like rapid air movement, and get blown off-course. This also helps to keep you cool, and is excellent where there is no A/C.
Do a search for the World Health Prganization (WHO), and look for Malaria to find out if Palawan is currently a high-risk area. Ask your doctor about which anti-malarial to take - some people have reactions against some kinds.. Don't also forget that mosquitos are the carriers of many other kinds of diseases, so using insect repellant is a kind of prevent-all
Hope this is of use to you