I am currently in the process myself. I cut the top for the following reasons.
- my kegs had handles at the top.
- Kegs have a rounded bottom (great for collecting trub or draining every last drop)
- It will remove that pesky dip tube.
For my mash tun I am looking at putting the ball valve on the bottom of the keg under the false bottom so I can drain every last drop out. I am also setting up a HERMs so I can spend the whole hour of the mash getting a clear run off.
If i were going to do it again, I would cut the bottom out of the HLT and MT and then use the factory spear opening as a bottom drain. i like having the side drain on my kettle so that I can leave some of the hops and trub.