I think that asking the locals might be fine for a local lake you have never fished before.
However, if you're planning a trip to another state and on to a big lake then I would strongly recommend a guide. They know the lake and the seasonal patters etc
Think about how you do on your local spots compared to the new lake you just found.
I have hired several guides over the years. I had some bad experiences and some good ones.
Here is an article I used when planning my last trip to BigO. I think it covers everything. Most IMPORTANT is know what you're getting. Once you know then there is no surprise.
http://www.guideservice.com/select.htm