When you upgrade the firmware on any device...you risk bricking it. It is very unlikely but it does happen. 1.5 gives you the best of both worlds trim and "ITGC". 1.6 just further refines it. The big thing is...you can not go from 1.4 directly to 1.6. You need to flash to 1.5, reboot THEN flash from 1.5 to 1.6.
Here is where you can get 1.5 and 1.6 firmware.
downloads section
If you are running on windows 7 with the TRIM enabled 1.4 firmware (and NOT 1.41) then the benefits are only minor. BUT Id still do it every little but helps. If you are running an older OS...its a no brainer. Do it. Same with a win 7 and 1.41. I ran 1.31 (even more aggressive itgc) as it is slightly faster firmware compared to TRIM enabled firmware..BUT they also are much harder on the NAND. ;)
Only you can decide but the chances of bricking are VERY slim. YMMV but i like to keep mine up to date.
Backup your data before you do as while it should be nondestructive...its always better to be safe than sorry. ;)