Last year I sowed tens of both mandarin and lemon seeds in hope to use them as grafting stocks in the future. But after reading postings here, I started to hesitate: as I've understood, lemon seedlings usually worsen the flavor of other citruses... And what about mandarin seedlings?

I have a similar question to this one. I have a bunch of the Sunburst seedlings I grew last year (mandarins) and I would like to know if I could use them as rootstock and how it would affect the taste of the grafted fruit.

Lemon was used early on in Australia as rootstock, but later disease problems led to a change to Rough Lemon, which is still widely used.
You may get some slight effect on fruit quality, but it may not be noticeable.
Cleopatra mandarin, is regarded as a rootstock to produce high quality fruit, and other "mandarins" have been used or investigated.
Apart from susceptibility to root disease, Sweet Orange is also regarded as a high quality inducing stock.
The main consideration for rootstocks is are they growing well under your conditions, their likely long term performance, and any compatability problems with the scion.

Thanks, pagnr! I understood that I should not worry much about using lemon seedlings as a rootstock.