Dragon #302 says:
Level CR
1 less than 1
2 1
3 2
4 3
5 4-5
6 6-7
7 8-9
8 10-11
9 12-13
there are issues ere. Somre creatures (such as the advespa devil) have the ability to do massive daage, but not to take it, some are amazing one trick ponies etc. You need to be careful what you allow and what you don't. The way we do it is to let the player use something, and if we decide it's too powerful at that level, or not good enough or whatever, then we change it.
Also note that newer WotC monster books have tended to address this by listing in the description (Fiend Folio) or in a table at the end (BoED) which monsters can be summoned by which SM spell. They seem alright to me so far, but I haven't playtested extensively or anything.