Should I beta test before a launch or post launch?

Beta launch is still a launch, and then relaunch and relaunch as many time as you need. But yes, calling a launch "beta launch" means you will get away with some bugs and issues :)

That totally depends on the product and the receptivity of your early adopters. I've done both and in some cases, even have no beta, but have beta [test] clients, with a view to building the technology as a co-production with their help. This means extensive, micro-Lean Startup activity which continually creates features and enhancements. A beta can be useful for a pipeline here. So your beta clients are early adopters and this stuff then goes into mainstream use after a while. Facebook does this and so do Hootsuite.

If you're launching a SaaS and assuming it'll be a paid service, you can do a beta launch, being explicit that you'll either give free service or a reduced price in exchange for feedback. When you've collected enough feedback and ensured your product works perfectly, then you can do a general launch with normal pricing.

We experienced that it really helps! Thee are more errors and improvements as you think.