Does Reporting Services lock tables?
FYI, unless specifically directed, all TSQL statements are presumed Snap-Shots.

1st Question:


So Think of it as your SSRS is looking at your data as if it were a View/Select statement.


2nd Question:


It could, pending on the volume of calculation (SSRS) and volume of users.  Think you would have to have well over 10% CPU/RAM utilization from SSRS and over 50% CPU/RAM utilization from general queries in order for you to notice an effect, albeit hardly noticeable (like 1-10 nSecs).