I think it is good to mix it up as often as possible. Personally, I find that it's very difficult for me to PR on an ME exercise for 2 or more weeks in a row unless it is a totally new movement. I have been switching my ME exercises every week to stay fresh and it is good for me mentally. I think a very good rule to follow is to cycle your ME exercises in a fasion where you hit a full-range movement then a partial-range movement.
Full-Range Movements: Reverse Band Presses, Decline Presses, Incline Presses, Dumbbell Presses, Illegal Wides, etc...
Partial Range Movements: Floor Presses, Board Presses, Pin Presses, etc...
You can use bands & chains with all of these movements. As far as cycling the bands & chains, the general rule I use is no more than two weeks with bands (choked down of course, reverse bands don't count as they aid in the movement) before a change to either chains or straight weight. The same thing for chains, no more than two weeks before a change to straight weight. I always go down in resistance. Mind you, none of this is set in stone. This is how I train. Some train with bands and/or chains all of the time but I can't as they really beat me up. If you are going to cycle your ME exercises every two weeks then I would stick with the same set-up for both weeks so you can try to PR on the exact same exercise. Pretty much, just common sense.
Example:
Week #1 - Floor Press w/ Chains
Week #2 - Floor Press w/ Chains
Week #3 - Close-grip Incline Bench Press
Week #4 - Close-grip Incline Bench Press
Week #5 - 3-Board Press w/ Mini-bands Doubled
Week #6 - 3- Board Press w/ Mini-bands Doubled
Week #7 - Reverse Band Press w/ Green Bands Suspended
Week #8 - Reverse Band Press w/ Green Bands Suspended
Week #10 - Rack Lock-outs - 3"-6" ROM
Week #11 - Rack Lock-outs - 3"-6" ROM
I think you probably get the idea. Rotate partial movements with full-range movements and cycle your band/chain/straight weight.
On DE day you need to be hitting 8-12 sets of 3 reps @ 45% - 60% of your 1-rep max with less than 45 seconds rest between sets. You need to be completing the 3-rep sets in less than 3 seconds. DE day is all about speed. You can use bands (mini's or light only) and/or chains on your speed sets if you feel physically up for it. It is probably a good idea to cycle the use of bands & chains on your DE day as well. When you hit your accessory work after ME & DE day make sure you focus on your weaknesses. If you can military press 315 lbs. but can only flat bench 250 lbs. then it's safe to say that your delts & triceps are not your weakness. So focus on your upper back, lats, and chest. These are only recommendations and a general guideline of how I do things. Remember, the beauty of training Westside is that your in control of your training. Westside training is the most effective when used intelligently and tailored to your personal needs. Good Luck!