Are transitions strips necessary from room to room if installing luxury vinyl planks throughout house?
Transitions strips are generally to provide a transition from one flooring surface to another; oftentimes because of a height difference. Sometimes they provide a threshold from one room to another where the flooring is the same but a limit barrier is desired, like from the living room to the kitchen/dining room. 

Transitions give visual cues and can change the design dynamics visually to a space—make it look bigger or more cozy, etc. 

If you’re using a single kind of flooring throughout and there are no height differences or underlayment/stability issues, you don’t have to install transitions. 

You do need to install base of some kind where the flooring meets the walls. Doorways, around corners and curved spaces require base and sometimes transitions, depending on the flooring and installation layout. 

Be sure to do a “dry” layout and determine where you need to make cuts, keeping in mind, the cuts should be made at the least visible part of the floor. For example, in a small bathroom, you generally want to use a full tile coming out from the wall or tub and make cuts at the threshold of the door.