PVC tape is a good choice for bundling wires - it is totally inert (the wires are typically PVC insulated anyway), it is flexible (to assist with installation and to allow for the cables themselves to expand and contract with temperature), it can withstand variations in temperature itself, it can easily be removed and replaced when service is required, and it is cheap and quick to install. It is the best choice all round. Plastic ties do get used to secure the bundles to tie points where support is needed (such as vertical support) but they would cause fatigue to the cables if used throughout.

There's a certain beauty to neatly bundled wiring harnesses. If you strip all the trims from your boot/trunk floor it is quite impressive. The photo in this thread looks quite messy because you have two bundles of wires joining at a 'T' so they are loose to allow for movement. If you could remove the surrounding trims to get some context it would look much better.