There is no standard practice for the appearance of Teppo Tai heraldry. Well-equipped units with daimyo who could afford it would have teppo ashigaru with the mon on the breastplate or on their jingasa. There are preserved samples of 'uniformed' Teppo tai armour with their guns on display.
Those who could not afford 'uniformed' armour, would have their teppo ashigaru wear kote-jirushi or sode-jirushi. These were pieces of cloth with the mon painted on them and attached to the kote (armoured sleeves) or sode (shoulder armour).
These men would fight together under a banner, some were unit banners signifying Teppo Tai or under the sashimono or uma-jirushi of their squad leader.
BUT, there are armies where the Teppo Tai wore sashimono. Sakuma Morimasa (1553-84) had his Teppo tai with tall narrow white sashimono with three red Sakuma mon. Sakuma Yasumasa (1555-1627) had light green versions for his Teppo Tai.
Ii Naomasa Lord of the 'Red Devils' had a Teppo Tai as hatamoto bodyguard for Tokugawa Ieyasu. These men wore narrow red sashimono with Buddhist prayers written in gold. These men are seen in the Sekigahara screens and their sashimono have been preserved.
As Teppo ashigaru started to dominate the battlefield, where they no longer just acted in a defensive role, and took the fight to the enemy. They had to be clearly identifiable on the battlefield.
Teppo ashigaru in some armies appear to wear mutiples of two, three, or even more sashimono sprouting from their backs. The Kyogoku had twin sashimono, one in black and the other in 'pink'? Date Masamune had different squads with different 'unit' triple sashimono.
The most extreme examples were Teppo Tai serving the Hojo and those of Osuga Yasutaka who served Tokugawa Hidetada during the siege of Osaka.
Some Hojo Teppo ashigaru wore very tall narrow sashimono, about the height of two men with one standing on one guy's shoulders. It had the Hojo mon on the sashimono.
Osuga Yasutaka had the Teppo Tai of Kuze Hironobu (1561-1626) wear tapering sashimono as tall as the Hojo's, with black discs (about eight or 12) going down them. According to the 'Seiki Shuzu' ( similar to 'O Uma-jirushi' in recording heraldry), these tall sashimono worn by the Teppo ashigaru also acted as unit nobori.
Archers attached to Teppo Tai appear to not evolve the same way as the aquebusiers. Continued to not wear sashimono as they would get in they way of the bow and quiver, relying in kote-jirushi.