Is using Steam Achievement Manager (SAM) against Steam rules?

Yes, it is against the Steam Subscriber Agreement.

It is against Steam Subscriber Agreement which you agreed by creating a steam account, buying a game or buying/selling something from/on the Steam Community Market.

You may not use Cheats, automation software (bots), mods, hacks, or any other unauthorized third-party software, to modify or automate any Subscription Marketplace process.

Steam Achievement Manager counts as a bot or cheat (TF2 Wiki considers it as hacking), if not just an unauthorized third-party software, therefore is against the Steam Subscriber Agreement.

If you use this software, your account may be terminated:

Valve may cancel your Account or any particular Subscription(s) at any time in the event that (a) Valve ceases providing such Subscriptions to similarly situated Subscribers generally, or (b) you breach any terms of this Agreement (including any Subscription Terms or Rules of Use).

In past, the TF2 "Announcer" took the items that were obtained through hacked achievements back for one week, which shows that Valve doesn't accept them. This also shows that it is not authorized by Valve.

Is it against Valve's policies/rules to use an achievement "cheat"?

There is actually nothing in the lengthy User Agreement that specifically mentions achievements. Sure, you could assume a specific line or section covers achievements, but this is just an assumption. The word "achievement" does not appear in the agreement last updated June 2015.

This question appears all over the Steam forums, and none of these direct you to an official word. That is because there is no official word. It simply is not specifically covered in the user agreement.

Will Steam ban you? Reset your achievements? Who knows? If history tells us anything, then no, you will not be punished. I have used SAM (Steam Achievement Unlocker) and know of many others who have used it. There are plenty of stories in the Steam forums too.

Scouring the Steam forums for mentions of punishments when using an achievement unlocker pretty much leads to discussions on VAC bans. These discussions are concerned with using the achievement unlocker on a VAC server. You will find plenty of people saying they use the achievement unlocker on VAC without ever receiving a ban, myself included.

The official Steam Achievement Manager site does mention that the only time achievements were reset was during the TF2 medic update. However, this did not stop people from using the tool to re-unlock the achievements.

Also in the same site, it states that games can now manage achievements server side. This means that achievement unlockers no longer work for all achievements (i.e. cannot unlock server-side achievements).