It is important that when you type Greek, you are using a proper Greek font, not simply the Greek characters you find in Times New Roman, Helvetica, etc. One problem with using such Greek characters is that they will not offer all the necessary diacritical marks.
Some Bible software programs come with their own fonts and possibly support for others. For example, BibleWorks comes with BWGrkl. It also provides an easy way to export (available to paste into a document) Greek fonts such as SBL Greek.
Important Note about Greek fonts
Be aware that your reader(s) might not have the Greek font that you are using and your Greek may then look like gibberish. To avoid that, embed the font, at least the characters used, into the document. In MS Word, this is done in “Save Options.”