Any good player knows that tech buildings are vital to their success. A player who controls 4 oil derricks, as opposed to a player who controls none, will have an initial capture bonus of $4000 (If you capture an oil derrick you are awarded $1000) and will also be making money from the oil derricks throughout the game. Oil derricks are particularly important to the Soviet player. Capturing oil derricks instead of building extra harvesters allows the Soviet player to keep on building rhino tanks; giving them a better chance of victory as a result. It is vital that you protect oil derricks and capture them early on in the game.
To protect oil derricks in an early game situation just build a pillbox or sentry gun next to them. You may think "But that costs another 500, and my engineer cost me 500 - which means I'm back down to nothing because capturing it only gave me 1000". Don't think that. Oil derricks supply you with regular income throughout the game - and pillbox/sentry guns are vital to stop enemy engineers simply walking into the oil derrick and gaining the 1000 capture bonus.
The key to taking control of the oil in a game is to capture the oil derricks early on in the game; before your opponent has the chance to. I send out dogs to guard the oil derricks before i send engineers - "why?" - because the dogs eat the enemy engineers or any other infantry that he may send. Obviously any good player will probably send dogs out as well, yet it is still a far better idea to send out dogs before your engineers.
Obviously there are other tech buildings apart from the oil derrick that can play important parts in games. Airbases are the second most important civilian structures in my opinion. I see little use in the hospital or tech depot, they are only on a few maps, and do not usually play large parts in the game.
The basic ethic though is this: capture and defend any tech building - and do it quickly. You must send out dogs to protect your engineers and to defend the tech buildings from enemy engineers. Remember, tech buildings have more uses than just the ones they were created for. For example they allow you to build off them, so if you capture an airfield in the middle of an ore field (e.g. on South Pacific) then it is a very good idea to build a refinery next to it, as this will boost your income considerably. Another tip, if you are the Soviets and you capture a nuke plant, why not sell it? This will give you extra credits which will help supplement the cost of the rhino tanks that are used for the Soviet rush.
Summary: Protect any tech buildings that you may have with at least a pillbox, sentry gun or dog. If your enemy manages to capture the tech buildings before you then you simply must destroy the tech buildings or capture them off him. If you allow him to keep the tech buildings then he will make extra income throughout the game which could prove vital. Tech buildings are important - so do not ignore them! Capture and protect them; their benefits can be enough to turn a game in your favour in a lot of cases.