Inherent Biases
Something to bear in mind is that you can’t record everywhere - not in people’s back gardens, nor in most fields, nor in the greater part of woodlands - so recording will introduce biases. Recording at greater intensities will exacerbate the biases - recording which grid square a species in present in will give biases to urban areas, and rural areas with good footpath networks, and country parks. Recording at higher resolution exacerbates this. Imagine dividing a grid square into 100 hectares. Locally to me I can access roughly 20 to 100 of those hectares, the upper end being wholly built up areas, but I don’t have to go all that far (about 20km) to find grid squares which are inaccessible, and I could only record large plants visible and identifiable at a distance, or none at all. Recording to grid square level is not too bad at mapping the overall distribution of a species; recording to hectare level results in mapping accessibility rather than plant distribution.
Personal Policy
My personal general rule for unvouchered recording (no physical specimens nor photographs) is one record per species per grid square per year, with repeats allowed on subsequent days for different sites and different flowering status. Multiple records of rarer plants are taken to allow them to be refound, and to allow recording of rarer variants of species.
Discussion of example taxa
At the one extreme, I’ve seen fields covered with Urtica urens. Recording every plant there would clearly be ridiculous, as well as impracticable. A practice that is recommended, but which I am mostly guilty of not following, is to record the number of plants at a site, switching to “many”, “hundreds”, “thousands” at some point.
At the other extreme, recording every amenity street tree (mark them as cultivated in iNat) in an area seems a not unreasonable project. For amenity shrub borders I would record the species (also mark as cultivated) present in a border/set or borders, rather than individual plants, possibly with duplication if a grid square (monad) boundary is crossed.
In the case of bracken, what is a plant is unclear. Bracken is rhizomatous; while shoots look like individual plants they grow from a common root system, and quite expansive patches of bracken can be clonal. Recording the location and estimated size of each colony of bracken could be done; that allows investigation into the ecological aspects of distribution, rather than just the geographical aspect. For Japanese knotweed people are probably interested in the location of every colony - for control purposes - but there is one country estate garden I know which has clumps scattered around the site, and I haven’t gone to the extent of recording every clump there.
A moorland full of heather has even more plants than a field of Urtica urens. One country park near me has small colonies scattered around it, and I have gone so far as to record individual colonies, rather than just grid squares. But when you get to a moorland, field enclosure or grid square for unenclosed moorland seems a reasonable resolution.
In my neck of the woods rowan mostly occurs as bird sown seedlings probably originating from cultivated plants. Plotting the locations of cultivated and bird sown plants could potentially be used to investigate seed dispersal. (A related species, Sorbus croceocarpa, occurs as old trees in a number of Victorian parks, but presumed bird sown specimens turn up great distances from those locations, so I suspect that a project with rowan wouldn’t work out - you’ld be investigated establishment rather than dispersal.) But rowan also occurs as wild populations in the understorey of birch-oak woodlands, and also moorland streams and other moorland sites protected from grazing. A woodland could potentially contain 100s of rowans.
Tradeoffs
Finally, you have to take the time cost into account. In the summer I can generate 100 unvouchered vascular plant records in a hour (20 in 5 minutes, but diminishing returns kick in), possibly 120 to 150 if I picked the right location (and maybe 300 if I automated GR recording and allowed duplicates); with the iNat app the number is about 20 (if it doesn’t lose internet connection) and with the Seek app maybe 40 or 50. Recording particular species at high intensity means compromises with recording more species, and recording a greater geographical spread.