boosting xp will raise your game stage faster, I am assuming, bringing on tougher hordes etc quicker, perhaps faster than you would want in terms of getting ready for it (resource gathering , building a base...)
I am picking that not boosting it is would be better for the tower defence types and boosting would be more fun for the 'insane level is too easy' types.
or even don't boost xp if you want a more paced saga of a game and do if you are in a hurry to get to the 'end'. I do play both ways, rushing can be quite exiting as you quite often get caught short and things can get tense, but I also like the grim satisfaction of being safe (ish) in a fortress and smashing the hordes quickly and efficiently. and brutally. and set up humorous ways to wipe them out.
seems to me this is a very nice choice to make the game play the way you want.
personally, I normally have heaps of unspent points, either because the game is going to plan and I haven't finished this part (say making a farm) , or I have found a chainsaw or auger and am going nuts gathering as much resource as I can. often its 'damn ive levelled again - that will makes the zds tougher" rather than "yippee I can max out skill x"
unless it is unlocking the mag 44 round and then it is definitely "yippee!!"
- - - Updated - - -
After giving it some thought I've removed XP bonus from INT. You can craft faster and higher quality items with more INT. There will still be consumables that give you a short duration XP boost, but we don't want min maxer delight.
damn! please read my post from a few minutes ago :-(
... looks like it joined them together.