The floor should be as level as possible. Otherwise, you will put unequal stress on certain parts of the pergo, causing it to buckle and possibly crack. Even a small bump can do it over time because it creates a surrounding perimater of unsupported floor around it. The underlayment can account for some of it, but ideally it should be perfectly level to extend the life of the floor.

You should use a glue stripper to strip the glue. It does the work instead of your back. JASCO is one of the best, but also the most dangerous/toxic. It is available in gallons at Home Despot in the floor and tile department. Be sure to use chemical-proof gloves (also at H.D.) & eye protection.