There are a couple of criteria that it is useful to your queries to ensure that you get the most accurate data possible. In both cases, you'll need to use the more options button.
Filtering out duplicates
To filter out duplicates, click on more options and then choose options > Filter duplicates
Make sure filter out duplicates is selected
The filter duplicates options references duplicates and tagged duplicates. Find out the difference between duplicates and tagged duplicates.
Filtering for validated records
To filter for validated records, click on more options and choose validation > validation state.
Select Accepted or unchecked from the dropdown.
Locality (precision)
Under more options > locality > precision, the drop down menu lists many options for minimum or exact grid reference precision. This allows you to narrow down the precision of records you want to view or summarise.
Grid reference and vice-county validation
Under more options > validation there is the grid-ref and vc validation option. This checks that the vice-county listed for each record matches the actual vice-county boundary. There are various options to choose from:
In many queries, it is good practice to include a clause that restricts records to those where grid-reference and vice-county match, or perhaps matching or within 1km of county boundary, to exclude records that may have a wildly incorrect grid reference, and which have not yet been corrected.