drtweak :
Look up your motherboard. Somtimes the 1x slots are only PCIe 2.0 while the x16 are PCIe 3.0. if that is the case I would then use the x16 slots first before the 1x slots this way those two will get twice the bandwidth as the 2.0 slots. If they are all 3.0 then it will not matter what ones you use. Maybe use the 1x slots this way you have the x16 open for anything else down the road.
Nice one for the info, Dr Tweak. Much appreciated. My MOBO is a MSI 970A-GAMING-PRO-CARBON (link below). Just checked its PCIe details and it says the following:
2 x PCIe 2.0 x16 slots (support x16/x0, x8/x8 mode)
3 x PCIe 2.0 x1 slots
So I guess that means they are all 2, not 3.
The reason I originally asked this question was because I am trying to troubleshoot a problem I am having with my GPUs and Display (see here: http://www.tomshardware.co.uk/answers/id-3437044/single-gpu-mining-rig-monitor-worked-fine-added-gpu-monitor-stopped-working.html). If you have any suggestions for that problem (even left field ones - I am at my wits end with it!), that would be swell! My mate mentioned something about a possible need for xfiring/using SLI cables. Isn't that for old cards, not nice new RX480s?
Also, do you know if is it ok to connect a x1 shaped connector into a x16 shaped MOBO slot?
https://www.msi.com/Motherboard/970A-GAMING-PRO-CARBON.html#hero-overview