As long as you're using condoms properly, it doesn't matter what sexual positions you and your partner are using. Condoms, when used properly, are quite effective at preventing pregnancy. In order to make sure your condom works properly, you want to make sure that it is not expired, has been stored properly, is applied properly, is used for ALL genital contact, used with extra condom-safe lube, and that you hold the rim when withdrawing. You can find out more about properly using condoms by checking out our article Condom Basics: A User's Manual. As long as your condom does not fail, it should provide excellent protection against both pregnancy and STIs. Remember too that sperm are well equipped to swim on their own and they get an extra boost during ejaculation, so if your partner's genitals come into contact with ejaculate (or pre-ejaculate) there would be a pregnancy risk regardless of position. While there are some indications that certain sexual positions may assist in conception for couples actively trying to become pregnant, as long as you are using a condom, all the sperm will be contained so position is really a moot point.
Like any form of contraception (aside from complete abstinence), condoms are not 100%. With perfect use, they have a failure rate of about 2%. With typical use, the failure rate is more like 15%. If you and your partner are wanting additional protection, you can always back-up your condoms with another form of birth control, either hormonal (the pill, patch, ring, injection) or non-hormonal (like a diaphram or Lea's Shield). Your partner could also utilize the fertility awareness method, which can then be used by combining condom use with periodic abstinence. Also, should you have a condom failure, you could use emergency contraception.
You may also want to check out the following articles for more information about choosing and using contraceptives and other topics: