Pretty much what Jenn said. It really depends on the country, the amount of money they have, what techs they have or may need, and the board situation.
Generally I want Germany the US and Japan to have at least 2 researchers at all time. However I rarely go over 4 unless I specifically want something and have a high probability of getting it (ie. I have 4 of the other techs on that chart) or it can deliver a crippling blow if achieved. Normally with Germany I purchase researchers 2 at a time, with the US 1 at a time, and Japan really just depends on what I want. Normally I have purchases in mind for Japan and any surplus cash past this is what I use. Also Japan is probably the country I care least about getting tech with other than probably Long Range.
I find that Italy, Russia and the UK can find the money to keep 1 researcher around and sometimes with Italy 2. I have even spent Italy’s initial 10 IPCs on researchers. Anything on chart 1 can be made to work for Italy without having to get overly creative.
I rarely have problems getting techs because in most situations I am investing money in tech to some degree with some of my countries either Axis or Allied every turn. Again like I said I do normally cap my researchers at 4, because I feel at 4 I a very likely to get a breakthrough either that turn or the next so I will fore go the expense of additional researchers.
I prefer playing with techs as I feel they make the game much more dynamic and interesting. I also do not fell any 1 tech will automatically win the game for you to included the highly touted US round 1 Heavy Bombers.