Once you configured SSRS with integration mode. If you create new web application and want to use SSRS there. Every time you have to go to SharePoint Central admin and in general Setting you have to activate feature from there (for new site collection under new web application). We understand we can go to site collection level and activate Reporting feature from there. But also need to require to go central admin and from SharePoint Central Admin > General Application Settings > click Reporting Services Integration you have to activate feature from as well. Why this is require. Because if you see after doing central admin steps if you notice it will show following message.
Access for SSRS service account to farm successfully added.
But one thing will always come in mind. Why every time needs to go there and do centraladmin activity every time. If we want to do for only new web app. Its true. You can do for new web application for following Power shell command. In this way it will not impact other web applications which are running previously.
Powershell command is:
$SPSite= this is thesite collection url where you want to give access to SSRS service account.
$webapp = Get-SPWebApplication $SPSite$ApplicationAccount = Service Account of SSRS.
You can follow other way is go to central admin and select Web application from application management and add Service account with FULL Rights under user policy setting.