Should I pre-tear my toilet roll before rosh hashanah starts, like I do for shabbat? I left it a almost too late to ask but...I am Unsure.
Halachically, tearing paper on Yom Tov is the same as tearing it on Shabbos. The questions involved are the same. However, they now sell precut toilet paper which prevents the paper from clogging the toilet. The main difference is that regular tissues are deliberately made not to fall apart in the water of the toilet bowl, while toilet paper is designed to do so (so it will not clog).
In שו”ת מנחת אשר ח”א סי’ כט אות ב the Rav seems to say that it is מותר to tear toilet paper on Shabbos. It that his שיטה למעשה?
The Rav’s intent there is that it does not involve the melacha of מחתך. However there would still bea problem of קורע. In the above tshuva, there is no קורע issue as the tearing is totally destructive. However with toilet paper the tearing creates a useful item which is prohibited. There is a dispute in the Rishonim if this korea, which is productive but not to prepare for sewing, is a Rabbinic or Biblical prohibition.
Rabbi Ostroff discusses the matter.
What about tearing toilet paper when there is no option?
Obviously one must prepare toilet paper before Shabbos. The problem arises when one has depleted the supply of tissues, torn toilet paper etc. or one is in a place where there is no pre-torn paper.
For obvious reasons we will not discuss the various technical options that are on hand when there is no toilet paper available, but when those options are exhausted and/or one’s only practical option is uncut toilet paper, the following is the correct procedure:
One should tear the toilet paper by resting one’s elbows on the sheet of toilet paper and tear it with one’s elbows. This is called tearing kilachar yad – in a backhanded manner, and is only an issur d’rabanan. 
Where do we find that one may violate an issur d’rabanan in such a case?
There are two sources. The Rama says  that one may carry stones (olden day toilet paper) from a reshus hayachid (a private domain) into a karmelis even though one is violating an issur d’rabanan. The basis for this heter is that Chazal did not institute their prohibition when it confronts a person’s dignity - 'kavod habriyus'.
The other source is the Mechaber  who permits one to raise a stone that has moss growing on it even though raising it from the ground is a violation of an issur d’rabanan.
This therefore is the basis for this heter.
Does it make a difference whether toilet paper is torn on the dotted line?
In order to answer this question we must, in a few words, familiarize ourselves with the melacha called Mechatech – tearing or cutting to an exact size or measurement.
'Mechatech' is the melacha of cutting or tearing an item for a specific purpose or size. An example of this is a gemora which says that one who purposely breaks the feather between the soft part and hard part has violated Mechatech. The soft part would be used for down and pillows and the hard part would be used for hat manufacturing. Since one is particular about breaking the feather at that exact point, it is mechatech.
Based on this and many other proofs we can determine that although tearing toilet paper involves Kore’ah and maybe even “making a k’li”, tearing on the dotted line is probably not Mechatech. Rav Shlomo Zalman Auerbach explains  that since one only tears on the dotted line because it is a convenient manner of tearing the paper but not because one is particular about the exact size of the paper. Proof of that is that each paper company has a different size paper and nobody buys paper because company A has paper size B.
This means that one may only tear paper backhandedly, as mentioned above, when there is no other feasible option, but one need not be particular about not cutting on the perforated line.
However, we do not find this opinion mentioned in other poskim and therefore, if possible, one should avoid tearing on the perforated line.