RGG responded literally as I was typing.
The tutorial Lex linked to is for hams that will both receive and transmit SSTV. Its stance of not using auto slant is solid advice if you are going to both RX and TX. But from a purely receive application there is no reason not to use auto slant, in fact it is preferred.
My advice for people receiving only, do the alignment as defined in the tutorial, and after complete then select Auto Slant as circled in the 2nd image on that page. If you do not select Auto Slant then anyone who transmits with an incorrectly adjusted slant will appear slanted to you. Having Auto Slant not de-selected is great if everyone is correctly aligned, but if anyone is not then Auto Slant can save an image.
T!