It depends largely on how you want to play your thief. As a support trapper you will want poison, acid and maybe antiresistance. A stock standard damage trapper will want Cut and Bomb, Acid as high a level as you can manage and perhaps poison.
You can of course swap level for cut for levels in spike or crush, but since they do about the same damage there is little point. Assuming you get far enough, Tab and Blow might be worth a look, but by that time you will probably have a feel for what you want.
basically what Wolind says but 1 thing to add:
after the recent skill update crush and spike do at lv1 on average ~30% more damage compared to cut parts. However its not clear yet what it would look like at lv7, if those 30% would stay or not. Addionally there of course is the far higher level req for those, probably preferable to do with a respec when hitting 30+.
(some values regarding this at my blog, updated a bit ago)
i think these skills needing more level to use, means make more damage than cut but now cut deal more damage wen i use cut parts before was dealing with 145 of weapon damage like 2900 on hit with variations. now cut parts with 145 weapon damage deal like 3700 with variations i mean skill on 7
but bomb before deal 4000 now deal like 3500 its nerfed now