Ladies,
For me, as I talk with families about what will be best in the situation where a student is headed into high school, but is coming from a younger HOD guide), it always ends up to be placement that I go back to again and again. This is what makes the decision such a unique one for each student, because each student's "best" placement will be his/her own!
Placement becomes trickier as a child gets older, because it isn't always as readily apparent where a child fits skill-wise.

Maturity also begins to play more of a role in placement, as does independence, and work habits. A child heading into the high school HOD guides will also have to be committed to a longer school day. Each of these areas will play a huge role in a student's success and overall feelings about high school. So, all of these areas have to be weighed. Then, there also begins to be the state requirements and college entrance requirements to be met as students head through their high school years, so all of these areas become a part of weighing correct placement too!
So, I usually end up going back to the placement chart first to gain placement knowledge (paying the most attention to the first page of the chart) with the 3R's heavily in mind. Next, I weigh the science as kiddos are heading into the high school years. After that I begin weighing the other deciding factors: maturity, independence, work habits, commitment to longer days, and state/college requirements. Usually, by that point I have a clearer picture of what the "right" placement for that student might be. Then, I recommend bathing the decision in prayer and waiting on the Lord to see if He confirms the decision.
I agree that while MTMM is a wonderful guide, in the end the decision of whether to use it or not really comes down to the best placement all around. However, in placement it is very wise not to overlook maturity, work habits, independence, and commitment to a longer school day! If a student is needing extra time in any of the 3R's, I would never bump that child forward past a guide. If a student struggles or is a bit behind in math, I would definitely keep in mind then how much time math will add to a child's day as math gets harder as you go up! This additional time needed to complete math each day, would make the high school guides much longer than we intend. So, I wouldn't bump a child struggling in math forward either. I would also never bump that child forward in the sciences (as the sciences become driven by math skills in high school). If a student did not have strong, independent work habits, or a willingness to commit to longer school days, with an increased work load (then I wouldn't bump that child forward either, or you'll be dragging him/her along for years to come).

If a student has health issues or family issues or commitments in many outside areas, I would make sure to balance school accordingly so that it doesn't add additional pressures that would overwhelm the student.

I could go on, but you're getting the picture that placement advice is affected by so many personal factors! This makes it impossible to give pat answers.
So, for those of you who are trying to make this decision for your own student, you may wish to each post a thread about your student and his/her skill-level placement chart-wise and let the wise ladies on the board talk through placement with you. Dialoguing specifically about your student may really help! Or, if you'd rather not visit about your student on the board, simply reading others posts may help. If you get a chance to come to convention, we can personally visit with you about your child's placement. No matter which route you choose to go, the best advice I can give you is that you may just want to treat this as if you are brand-new to HOD and are seeking placement advice for your student. This may make it clearer to you as to what you should do! I know it makes it clearer to me as I help families.
Last, I'll say that usually when looking at high school looming several years ahead on the horizon, and wondering whether to fast-forward a child up a guide earlier in the guide sequence (i.e. like in the guides before Rev2Rev or MTMM), I'm more prone to wait to make that decision until the child is right on the verge of high school. This is because kiddos can change so much in just one year and also because it is tough to back up a guide if it doesn't go well at that point. So, this is typically the reason why I don't advise jumping a child past a guide too early in the sequence (especially once a child's placement seems right). However, just as is true anywhere along the HOD path, if a child seems like he/she is not well placed in a guide, then it is always wise to reassess and move that child up a guide to get a better fit.
Blessings,
Carrie