It doesn't really matter whether web/app server is behind LB or not. Actually in vast majority of production environments web/app servers are always behind LB, so this is pretty much common situation. What matters is where you put your RUM probe in such deployment.
If you put your RUM Probe before LB, RUM probe won't be able to monitor individual servers, as in this case for RUM it will be only one "server" - LB itself. So the best practice is to put RUM Probe behind LB as well, so that RUM could recognize every individual web/app server. However, in this case there may be another problem - "client" for RUM would be LB, not real uers. In order to avoid this problem LB should be configured to support X-Forwarded-For header that will keep original client IP. RUM Probe knows how to work with XFF headers.