To get towards Stuttgart and Ulm it is best not to stand next to the direct "Schnellstraße" B27 between Orschel-Hagen and Römerschanze, but go to the other side of the city. If you're far away from the city center take line #1 (towards Orschel-Hagen) or line #7 (towards Efeu) to the stop Karlsstrasse/Fernmeldeamt. This busstop is the place to stand as it is at the intersection which leads to the B28.
West towards A 81, Tübingen
Esso petrol station close to train station "Reutlingen West". Easy to get to Tübingen, A81 possible.
South towards nowhere
If you want to go south or to the Swabian Jura you should stand at the busstop in the Lederstrasse. This is where you can also find the ADAC or the Lois. It it in the city center on the way towards Pfullingen and Eningen. If you can't find it, ask locals, they usually know it.
If you ever need to go somewhere by bus you can find the overview of Reutlingen's bus lines here