[CQ-Contest] Pileups and calling freqs.

[DXer]

You guys must be reading my mind. I was thinking about asking, but did 
not want to start a thread, but now I'll. LOL

Long time ham, but very green on certain aspects of contesting.

I intuitively call a bit off frequency. My question is: How far can it 
be? My guess is anywhere within the filter BW. For example, if the other 
station is using a 250Hz filter, anywhere up to +/- 125Hz? Is this correct?

Also, for the very first time I saw a station that appeared to be 
purposely shifting between CW-U and CW-L. It threw me off at first. I 
thought my receiver was acting up. Is this a commom technique?

Thanks in advance for the education.

73 de Vince, VA3VF