Starting something new and while recording something not related to the project tempo, I usually have the '‘Toggle time base’ button of the track on which I am playing set to ‘Linear’. After the take, I adjust the project tempo to it in the transport panel, then toggle this option to ‘Musical’ for eventual further adjustments : quantize, editing, etc.
The main usefulness of this setting is that it allows me to freely play, even without having planned to record anything (thanks to the ‘Retrospective record’ feature), then, if I feel that I could get something useful of what I just performed, to retrieve it and adjust the project tempo to it without hassle, as increasing/decreasing the project tempo rate doesn’t affect the take timing under the ‘Linear’ mode of this switch. This is one of the Cubase features that makes it so intuitive to use for me.
More details in the Operation manual, p.74.