there were a few threads on that when the alpha rules came out, not sure if people have gathered more practical experiences.
viewtopic.php?f=129&t=8134
viewtopic.php?f=129&t=8011
the one line summation, use the 20 when you want more character development over time and take for example 18 for short games and one shots, or if you just want a slightly easier start.
Additionally, the rules got a short section on tweaking the TN calculation on page 18, that basically covers that.
TWEAKING THE TARGET NUMBERS
Calculating TNs by subtracting the Attribute scores from
20 results in difficulty thresholds that are appropriate to
medium to long campaigns, as they present a challenge
to inexperienced individuals, but hold up well as those
same characters advance in proficiency. For shorter cam-
paigns or one-shot games the players and the Loremaster
may agree to derive the TNs by subtracting their Attri-
butes from 18 instead.
The starter set uses 18 without specifically mentioning the option.
also nothing stops you to start with one and change it over time. It is basically an easy way to adjust the difficulty depending how things are working out and what kind of tone people prefer.