Do I want an index if all I care to query is field not null

SELECT id FROM accesses WHERE token IS NOT NULL;

The perfect index for this specific query would be a partial index:

CREATE INDEX accesses_foo_idx ON accesses(id) WHERE token IS NOT NULL;

The index condition is the important part.
On top of it, since you only retrieve id which is covered by the index, you can get index-only scans out of this (if the table is vacuumed enough).

Effectiveness grows with the number of rows excluded from the index this way (and also with the number of columns in the table). I.e.: especially beneficial for few non-null values in token (in a table with many more columns).

Related:

• Unexpected Seq Scan when doing query against boolean with value NULL
• Index optimization with dates

You could always test this against your data and possibly use some sort of query analyzer.

Since there are a lot of factors for "common considerations about indexes," you should consider things like: data size, data type, and inserts, updates, deletes and not just selects. How often do you run the query? Is this an occasional check for blanks? How often does this table's data get modified? You can compare and contrast having an index in your system to see if it makes sense.