When is it ok to follow Beis Shammai?

The gemara in Brachos 11a says that, at least according to one opinion, you are not yotze krias shma if you go out of your way to pasken like Beis Shammai and lie down and you may even be chayav misa (I'll assume that this one definitely isn't literal). The mishna later in the mesechta on daf 51b says that if someone forgets to bench he doesn't have to go back according to Beis Hillel. However, there the gemara says that if you do like Beis Shammai then it's good.

I think that the simple explanation is like Tosafos implies in Sukka on daf 3a. By benching, even Beis Hillel agrees that it's better to go back but it's just not necessary. However, by krias shma there is absolutely no benefit to lying down. It's no better to lie down while saying shma than it is to stand up.

I thought that I remembered seeing somewhere once that here if you pasken like Beis Shamai you need to darshen the pasuk differently than Beis Hillel so even though it seems like it's only a chumra here of Beis Shammai but it's based on a faulty (acc. to Beish Hillel) understanding of the pasuk. Whereas by benching there is no machlokes on how to darshen psukim - it's just svara.

I'm sure there are other explanations also but that's all I know. As far as I know this is the only place that the gemara says if you follow Bais Shamai you're not yotze and presumably it's the only place that it's true when Beis Shamai seems to just be saying a chumra.