When it comes to highlighting the keywords in the copy in terms of the effect they have on search rankings.
I prefer using bold only for subheadings and not within paragraphs. Therefore, I was thinking if italicising the keywords in the paragraphs will have the same impact as bolding.
I would not have thought so. Bold is used to make words stand out whereas italics are for quotes and other snippets of text. If anything italics might indicate you had copied the text from somewhere else and were simply quoting it in your content.