At the lower levels, the damage range for the weapon is not as greatly influenced by the % as it would be in higher levels. Guessing here, cause i cant remember the stats of a 16 sos bow, but the difference in phys between 0% and 100% would probably only be 8-13 points or so, which you would not notice a huge difference from. In the higher levels, the %'s are extremely important. Ive been messing around (well, wasting money is more appropriate

) with my sun bow. At +5 and 0% phys, it has a minimum phys of about 875 and at 80% phys its at about 925. That is a pretty considerable jump that you will be able to notice quite clearly.
So in the end, dont worry about working on the %'s for lower level (5d, or maybe 6d and under) gear and weapons cause the money you will likely put into it wont give you a drastic change in damage but at higher levels (7d and up) you should greatly benefit from it.