In my opinion it depands a little on the kinds of traffic you have on your network and how you need / want to give priority to that kinds of traffic.
Let say you have: 5 kinds of traffic you need to handle : VOICE / VIDEO / Remote Desktop Protocol / VPN-traffic and normal internet traffic.
First you need to make a priority list out of them. See my example
Prio 1: VOICE
Prio 2: VPN traffic
Prio 3: Video
Prio 4: Remote desktop traffic
Prio 5: Internet Traffic
Now you can start assigning queues to your traffic, Junos has 4 COS profiles defined by default (See the explenation below)
Just an example:
Prio 1: Assured-Forwarding
Prio 2: Expedited Forwarding
Prio 5: Best Effort
Let me explain it a little:
For SRX and J-series devices you have eight queues built into the hardware, four queues are assigned to four forwarding classes by default.
Queues 4 through 7 are not assigned by default. If you need / want to use them you have to create forwarding classes yourself.
In a default setup all incoming packets, except the IP protocol control packets, are assigned to the forwarding class associated with queue 0. All IP protocol control packets are assigned to the forwarding class associated with queue 3
Default assigned queues on J-Series / SRX
Queue 0 | best-effort (BE) | The Juniper Networks device does not apply any special CoS handling to packets with 000000 in the DiffServ field, a backward compatibility feature. These packets are usually dropped under congested network conditions. |
Queue 1 | expedited-forwarding (EF) | The Juniper Networks device delivers assured bandwidth, low loss, low delay, and low delay variation (jitter) end-to-end for packets in this service class. Devices accept excess traffic in this class, but in contrast to assured forwarding, out-of-profile expedited-forwarding packets can be forwarded out of sequence or dropped. |
Queue 2 | assured-forwarding (AF) | The Juniper Networks device offers a high level of assurance that the packets are delivered as long as the packet flow from the customer stays within a certain service profile that you define. The device accepts excess traffic, but applies a random early detection (RED) drop profile to determine whether the excess packets are dropped and not forwarded. Three drop probabilities (low, medium, and high) are defined for this service class. |
Queue 3 | network-control (NC) | The Juniper Networks device delivers packets in this service class with a low priority. (These packets are not delay sensitive.) Typically, these packets represent routing protocol hello or keepalive messages. Because loss of these packets jeopardizes proper network operation, delay is preferable to discard. |
Now you can create 4 more queues if you need them.
Have a look at https://www.juniper.net/techpubs/software/junos-es/junos-es93/junos-es-swconfig-interfaces-and-routing/default-cos-settings.html
Hope this helps a little