It's your call if you want to upgrade every single bit you can. On later levels reaching +5 is really easy. Also once you get a higher tiered weapon, start synthesizing the next tier of wrenches (Battle Wrench, and so on...) and the next tier, and the next tier, depending on how far you are on the game. That's because higher tiered weapons like the Battle Wrench will give more stat bonuses when synthesized on to your main weapon.
Tip: Build the forest town's river around it instead of through it since it doesn't look wierd. I kinda disliked the limit of stuff you can build/place on the forest town because I thought it could've looked much better if you can put in more stuff.
--
...then blame it to the cabbages