CR for me is a warning flag. Monsters with a higher CR than the players -may- be capable of one shotting PCs or -may- have abilities which could be impossible for players to deal with. But I then follow that up with actually checking the damage the monsters are capable of and look at what special abilities the monsters have. If there are no problems then I use them, if there are problems I consider the situation carefully. The deadliest fights I've seen are ones where the monsters have abilities the players weren't aware of. And ones where there were lots of monsters. Since 4th ed I never expect solos to actually be threatening.