Is it a good idea to go with no travel guide to Japan?

You can. Travel guides can be helpful in many areas but Japan is so safe that even doing the wrong thing will not get you in trouble. Getting lost might happen from time to time but it’s part of the fun. Japan is known for having strict etiquette but they are very forgiving of foreigners and you get a gist by observing others before doing something.

What I highly recommend is getting a good guide book. It will be a great help for many reasons:

  • Planning ahead is crucial to get the best use of your time and the best value which is important if you are on a tight budget. It is easy to spend significantly more when you arrive on-site if you are not aware of prices, you are not going lower-cost seasons and you are not taking advantage of tourist programs that are there to lower cost (such as bundles giving access to multiple sites, transport,etc). A good guide will cover all of those, just be sure that it is no more than a few years since being published.
  • Having the guide with you, even in ebook form, is really helpful. When asked for directions in places that use non-Latin letters in their common writing, this is extremely important. You can show a page when you ask for directions and locals will be much more able to help since most guides will show the anglicized name besides the name in the local language.
  • Guides offer images, maps, sometimes extensive paper maps and usually include transit maps for place that have massive transit systems.

The web can provide much of that information for free but it will be scattered and of various levels of reliability as most things online are! A guidebook costs little but gathers everything in one place, including tips and common phrases. If you have a smartphone, bring that too but support may be troublesome as I don’t know whether bands supported by your phone will work in Japan, particularly if you are on a CDMA carrier, so download everything possible in advance (maps, addresses of points-of-interest,etc).

My personal favorite guides are from Fodor and they have served me better than any other guides. They have a good amount of information covering typical tourist interest and their organization stands out which makes them easier to follow and refer to than most books. Ebook versions are available too if you do not want a hard copy due its weight.