Here's an answer I gave to a similar question recently, but make sure you get a woman to answer for you!

Tough and important question.

1. Depends on local office / region
2. Depends on client
3. Depends on your personal style

It's true that the trend is less and less formal, but this means different things in America vs. Europe and in Financial Services vs. Pharmaceuticals vs. Tech.

When I was at McKinsey, I typically asked my engagement director what the client "dress code" is before I started. When I was the EM, I made sure to tell each new consultant what people typically wore, and let them make their own decisions. You never want to assume. I once showed up to a manufacturing client in Montreal in jeans and work boots only to find that everybody from middle management upward was wearing formal dress clothes, some including ties, and I felt very self-concious in my scuffed work boots, which I had specifically intended would indicate that I wasn't new at this!

In the Toronto office, I noticed that people seemed to dress "nicer" on Fridays when more people were in the office.

You can always ask a female office manager, if there is one. She will undoubtedly appreciate where you are coming from and help you out!

One more point, it's ok to be yourself. One of the best and most surprising things I found at McKinsey is the culture of being yourself. Be professional, of course, but be yourself!

Hope this helps.

Allen