While I don't feel the need to modify the stock iOS theme, I do definitely like some of the tweaks, additional functionality, etc. offered by JB apps like NCSettings, Springtomize 2, StatusTweak, WeeTrackData, Zephyr, IntelliscreenX, BadgeClear, OmniStat, etc.
While I know that "bricking" the phone is a possibility, the much greater risk, IMO, is that of "loosing" your JB. The more likely scenario is that your jailbroken phone crashes and you have to do a full restore in iTunes. This will obviously cause the jailbreak modifications, Cydia, and Cydia-installed apps to be overwritten, i.e., you "loose" your JB. So to get back to where you were before the crash, you'll have to re-jailbreak and re-install the Cydia apps. (I'm aware there are some Cydia backup apps that can help, but this approach has it's limitations. The fact is , there is no "iTunes-type" backup process for your Cydia apps.) If you're only jailbreaking to install TetherMe, or some similar app, than I don't see that loosing your JB as that big of a deal honestly. However, the more Cydia apps you've installed, the greater the PITA factor of loosing your JB.
Now, with all that being said, I think the real issue for you is the download speeds you'll get through your cellphone service provider. The 3GS is capable of getting 3G speeds max (generally 0.5 to less than 1 Mbps), and depending on your provider/available service where you are going to be tethering, you may actually experience slower speeds than that. When you compare that to VZN FiOS speeds (15 to 300 Mbps), well let's just say you're talking about an incredible difference.
Your other problem, of course, would be your data usage while tethering. Various issues include whether you'll have to pay extra for a tethering option, getting throttled or having your account cancelled for excessive data usage, having to purchase a more expensive plan to cover your additional cellular data needs, etc.
To give you a feel for your speeds and data usage while tethered, why don't you JB your phone, install TetherMe or a similar app, and try it. My guess is that it's going to be painfully slow. If you're already on iOS 5.1.1, or after you update to it, you can use the Absinthe v2.0.4 JB at http://greenpois0n.com/. I followed their instructions exactly, and had no issues:
How To Use Absinthe 2.0 (if you’re unjailbroken):
1. Make a backup of your device in iTunes by right clicking on your device name under the ‘Devices’ menu and click ‘Back Up’.
2. Once your backup is complete return to your device and go to Settings – General – Reset – Erase all Content and Settings. This will make the jailbreak process much faster.
3. Open Absinthe and be sure you are still connected via USB cable to your computer.
4. Click ‘Jailbreak’ and wait…. just be patient and do not disconnect your device.
5. Once jailbroken return to iTunes and restore your backup from earlier. Right click on your device name under the ‘Devices’ menu in the left panel of iTunes and click ‘Restore from Back Up…’ then select the latest backup you created before. (restoring this backup will return all the content previously on your device ie, apps, photos, etc.)
6. Thanks for using Absinthe, enjoy your jailbroken iDevice!