Should I normalize?

That's what the Threshold control is for. Set the threshold for the level at which you want the effect to "kick in." This can be as low as you need it to be depending on the material and/or the effect you're after.

Say wha' ? 

It's safe to use it 

I agree with Jens. Normalize in Reaper is a non destructive action, and it only changes the gain of the take/item (the same as if you turn up the fader). So if you need more headroom, just normalize the item, and you will have a rest for giving more gain through the fader. If you have several parts, separated with splits, select all the items/takes, and use 'normalize selected items common gain' function, instead of normalize one. What the normalize selected items common gain does, is to higher all the takes considering all the separated takes as 'one take'. It's a good option when you normalize a sliced and edited drum part. In this case you would need to push up its overall gain without compromising the dynamic range, and in-context loudness of each shot. ZRG

Here is how I kind of use normalizing. First I only use 'Normalize selected items (common gain)'. Hard normalizing is lossy and destructive, common gain is not. When a recording (eg. vocal) is too quiet, I use this on one long take. Then I hit F2 and see how much it has turned up the volume. I decrease it by 2db or so then select the rest of the takes and increase them to the same common gain using the volume slider in F2 (item properties). While on the subject, when I have I start a mix I will generally turn all the faders down -6db. This seems to be a good amount to enable enough headroom when lots of tracks are played together. If the master is peaking I will sometimes select all the tracks (sends and all) and bring them down a bit with the mouse wheel (weighted aluminum freewheeling Logitech G9 scroll wheel is awesome).

No problem, dumant; it does seem confusing! REAPER's still awesome, though, if maybe a little quirky at times.