Summer tends to be cheaper due to the heat... but as some have already pointed out... the casinos are icy cool and you're not going to be outside all that much. (OK... when lying nearly naked by the pool you have to remember you're in the middle of the Nevada desert. Don't overdo it!) Definately check the various hotels' online rate calendar. Cheap rates are available from time to time throughout the year. The rates fluctuate wildly depending on if a convention is in town, etc. If you're flexible, you can choose a time with the cheapest rate. Now... let me whisper something in your ear:
STAY AT THE SAHARA HOTEL AND CASINO
Rates as low as $39.99 per night... on the strip. They are way to the north and a bit out of the thick of the action... but the Las Vegas Monorail stops at the hotel and will whisk you to the major strip action in mere minutes. I've been going to Vegas for years. In my opinion, these are the nicest rooms at the cheapest price you will find. It has a nice casino that is comfortable and friendly... the kind of place you want to come "home" to. Room service food is not expensive... pamper yourself. Book online for the best price... then ask politely for an upgrade at check in. At first they'll say no. Then you will get a upgrade. I love this place!
You'll find nice rates also at Imperial Palace, San Remo, Tropicana, Boardwalk, Barbary Coast, New Orleans and several other places. Trust me: they're dumps.