If your slam is expiring, it's worth delaying your BT for a bit. If it's not in danger of expiring, you can wait for the next cycle to use it. This is the general rule, not gonna discuss the double slam procs since those are half a gcd and you don't have access to those playing on FM.
Here is the exact quote regarding the optimal rotation:
"Every single possible situation can be described as a modification of the BT-WW-BT combination. Since BT will always have the highest DPS due to its low cooldown, it will always have the highest opportunity cost with delaying it. No matter what, you will always see the pattern repeat of BT-WW-BT, with some amount of slams or other abilities in between. Based on the positions of the Slam procs and due to Heroic Strikes and abilities casted before, this pattern will have variable differences in time between abilities based on when Slam is used and GCD crowding. When we talk about deciding when to use Slam procs and it's ramifications, we will always discuss any delays incurred on this BT-WW-BT pattern.
A frequently asked question is when to use Bloodsurge Slams and when not to. In the rotational basis, these Slams are used in places that do not delay the BT-WW-BT pattern unless the proc (the actual Bloodsurge buff) will expire before you can use it. There are only a few unique locations in the rotation where a proc can occur and expire later if not used. These locations in the rotational timeline can differ depending on the player's lag and reaction time. The following are the three locations for slam usage which optimizes the rotational basis:
After the 2nd BT but before the point where if Slam was hit, its GCD would result in DPS loss from BT/WW delay.
Before the 1st BT in the next period if immediately after a critical ability or Slam which trails the 2nd BT if it will expire before the WW GCD in the next period is finished.
Before the 2nd BT if it will expire before the 2nd BT's GCD finishes.
This will not only minimize losses from Bloodsurge being overwritten, but also minimizes any losses from delaying BT or WW when Slam needs to be used. All rotations adapt on this principle. In a lagless (and 0 reaction time) environment, following these rules will create four variants of the rotational basis. A 8.0s period, two 8.5s periods, and a 9.0s period which are shown below.
https://i.imgur.com/AEtXxeZ.png
An 8.0s period would occur if a Bloodsurge Slam was not available after the 1st BT nor immediately before the 1st BT in the next period. The 8.5s period can occur if a Bloodsurge Slam needed to be used before the 2nd BT per rule (2). Due to latency, reaction time, and human error issues, these perfect rotations always become broken or stretched. The 2.5 seconds after the 2nd BT's GCD finishes is especially sensitive. The player must be able to react to a new proc and decide if it needs to be used right away or if it can be saved for the 1.0s space after the WW's GCD."

