It's technically made to go either way, but laying on the back or angled back is actually best for the transport and plastic reel clampers (hubs) – this according to a Tascam rep from a 1989 H&S Recording review of the TSR-8.
I have mine angled back in a rack stand. Because the LED meters are on the front, having it completely horizontal is a little inconvenient for visibility, IMO.
Another thing to be careful of when angled or horizontal on any machine is cleaning the capstan. You don’t want alcohol to drip down the shaft, so don’t soak the swab dripping-wet for that procedure.