IMO. After the timer unit. General pressure can be controlled pre-timer. Post timer, individual feeder pipes can be regulated by simple tube clamps, available from garden centres and aquarium supplies.
After the timer. The drippers are meant to run off a certain pressure so check with the company that makes them. There are things that will affect the pressure such as the length of tubing and the size of emitters. You should have a filter set up inline and be prepared to have to take the emitters apart to clean them. Drip systems are great when they work but take a bit of work to keep them working. As far as a timer it sounds like a good idea but in reality, you need to keep your eye on things so you might as well turn the system on and off. Another thing to consider is what are you going to be watering. Perennials don't move so it's easy to place the tubing. Annuals on the other hand come and go so you can't really lay out the tubing to take care of them. Bushes need more water and not a drip so forget about them also how many plants are you trying to water? if it's a lot then a standard sprinkler will get the job done better. It does use more water but you can be sure that you are getting everything. And don't forget that a lot of plants need deep watering once a week so that the roots grow properly. A drip system may not work for them.