Looking at the numbers, monsters in monster manual 1 will look to be pretty dangerous for an equal level player character, one on one, at low levels (I remember thinking the first thing when I started to play 4e). It isn't one on one though, and typically players will be able to act more effectively as a team than the monsters will; especially when you include healing, encounter powers, and particularly the riders on at-wills that monsters won't have at that level.
What you can't cater for is chance, and if the monsters roll well and the players don't, then things can get unbalanced. The first three or four levels of 4e are much more prone to character deaths than any of the later levels (in my experience). You can't realy do much about that though, it's fairly inevitable given the way the game is designed.
If your players are new to the game, and aren't working very effectively, then it might be necessary to adjust the encounters a little, but I personally recommend reducing the hp of the monsters and not the damage they deal - even a relatively small reduction will have a marked effect if the players are focussing fire at all, and it's much more satisfying to win an encounter against a dangerous monster because it had few hp than wittling down a monster with lots of hp but little damage.
I don't think that monster manual 1 monsters need any change at all up until about level 5 or 6.
Where you will start to have problems are later levels and new material. To address both:
Later Levels: monster damage output (for MM1/MM2) does not scale with level (they do too little damage) and monster hit points increase too rapidly (fights turn into tedious repetition of at-wills with no risk or reward). If you don't like tweaking monsters then once you get past level 5 you should start switching, cautiously, to newer monsters.
New Material: not so long ago, WoTC changed the power of monsters. They increased the damage and chance to hit and reduced hit points (probably other stuff too, but I didn't really pay all that much attention). Unfortunately I don't think they playtested this enough at low levels, and for a group of new players, the more recent monsters are very dangerous. In my opinion they should be used with caution until paragon. Once you hit paragon though, you'll need to be using new monsters all the time (MM1 ones will not prove a significant threat at all), and once you hit epic you'll possibly have to increase even Monster Vault monsters to provide a significant challenge.
TL;DR - MM1 monsters are fine when you're starting out. Stop using them after a few levels and switch to later editions (MM3 and Monster Vault) once you're into paragon.