I do think your focus and state of mind is important and reflected in the readings. That being said I don't always do a clearing or grounding of my own energy before hand. In fact I rarely do intentional grounding, something I could probably stand more of lol.
Sometimes I will use the cards at my alter, in which case I do always do a ritual with candles, prayer, incense, sage, etc. with intentional clearing on myself. (usually) always clear the cards themselves prior to reading with at least tapping, sometimes other rituals - usually with things like crystals, the four elements, sun and moon baths, etc. - so lots of different things I do and just go with what feels right in the moment.
Often times, however - I'm reading cards at my desk in between working and just stealing little moments, usually still have candles and/or crystals around but otherwise there's cards sitting in the desk which I pull at random moments, hectic moments, if i feel anxious, etc - with no grounding or clearing. I do find that I get relevant and good answers with this method as well so it would seem that the special rituals or clearings aren't necessary. I think it's probably mostly about the intention behind what you do, what helps you to feel most connected and/or right for you, and for some people an extensive ritual helps them do that, for others I think it can just be a split second decision.
