Yes, different head heights will definitely affect flow rates.
I run one tap from my reservoir to the main 13mm supply line, off which run all my 4mm feed lines to the pots. With a sprinkler/jet inside the reservoir, there is enough back pressure from a decent pond pump to supply all the pots even with the valve (tap) half closed or more.
First I set my timer: always one-minute intervals and then 5-6 times per 12-hour period during summer, and about 4 times in winter (to reflect the reduced evaporation).
Next, I open the tap to ensure there is enough flow to each of the pots and - more importantly - that there is enough flow to prevent salt build-up at the 4mm feed line outlets, as slow dripping can lead to salt and sediment blockages at the mouth of the line. You need a bit of pressure out of the 4mm lines to keep them clear (I do not run line cleaners in my reservoir).
Finally, I have at least two 4mm lines to each pot. This saves your plants if one line becomes blocked while you are away. Having several lives to each pot also allows you to chop and change - you might have two lines going into one pot, and three into another that requires more watering (different strain, bigger plant etc).
So timer, tap and lines are the three ways I regulate nutrient to each pot.
Adding an extra line is also a way to compensate for different head heights that reduce flow to the higher pots.
These are the reasons why I use separate lines instead of halo rings.
