I have found that selenium only seems to affect the dark tones. I do not make any density adjustments for selinum toning. A slight kick in the blacks for me is a good thing, I have never seen selenium muddy down the shadows.
When printing with papers to Sepia tone, there are those who make the print slightly darker and flatter to compensate for the bleaching effect of Part A.
I do not change the print very much as I use a very diluted bleach A and then tone with part B. As well I then Selenium on top of this.
I believe that the Sepia toner protects the highlights and midtones and the selenium protects the shadows for greater permance. *Tim Rudman may want to jump in here*
The combination of light sepia and strong selenium on a cold tone paper is one of my favorite looks .
I agree with Bob that the combo of Selenium and sepia can be particulary beautiful, but the results do vary with different papers. Some papers, like Multigrade IV, show an initial cooling off of shadows which contrasts beautifully with a warming of highlights in sepia after just a brief bleach, giving a duo-tone effect. Others papers that shift tone markedly to brown in Selenium will give quite a different look when split sepia toned to the highlights - the difference between the two browns being less obvious.
Selenium works most obviously in the shadow tones at first and will increase their density (DMax). This has the effect of raising contrast by giving deeper blacks - highlights staying put. It will also cause a colour shift that varies a LOT with different papers.
Longer Selenium toning will show the colour shift moving up through the tones and eventually it will reach the light tones if toned long enough.
Prolonged selenium toning will also eventually, after the initial rise, cause a fall in Dmax again.
Sepia, on the other hand, tones the areas affected by the bleach stage. This is initially seen in the highlights and a short bleach stage will allow sepia toning only to light tones. A longer bleach will allow toning of light and mid tones and a still longer bleach will allow sepia toning of all tones - but with some papers the blacks can take a long time to go. They always will though, given enough time and strong enough bleach.
Re your question about compensating in the printing - it depends on a number of things: ...
How much the image can stand in increased contrast, which can be quite modest. This is often not necessary to compensate for, as it may only affect the deepest blacks. If there are important deep near blacks they might become harder to see if they darken though.
It also depends on the paper, as others have pointed out. And it depends on how long you tone for, as it is progressive (but see above).
It is my experience too that it may depend (more or less - according to paper) on the strength of the selenium as it may behave differently when very strong to when very weak (for example).
Tim