The problem is that the English language is heavily burdened with homophones, and those little suckers will trip you every time. "Bear" sounds the same as "bare" but they do not mean the same; unfortunately, "bear" is a real word, so it will merrily pass under spell check's threshold, and you will have Jack being "bear" for Ianto's benefit, which would kill the mood faster than a dunking in iced water. In the same vein, check the differences between "lie" and "lye" (big one, I promise you, and one wouldn't be anywhere as much fun as the other). 

This last one leads me to "lied" and "laid". I know that "to lie lazily in bed" is one of Jack and Ianto's favorite pastimes, but "they lied in bed" makes them... not gentlemen. English conjugation is complex and irregular verbs abundant. For example, the past simple of "lie" is "lay" and the present participle is "lying" (yes, the same as for "to tell untruth"; I told you it was complicated!) .  Meanwhile, "laid" can be used as a past simple, but it does distort the sentence somewhat. So... check them out before you use them. There are plenty of websites that are extremely helpful in these matters.

My advice is, a good beta helps, but if you don't want to go that route, become familiar with those words that are likely to trip you.  Your fiction will be the better for it.

Now, excuse me, I'm going to go find a good "Jack and Ianto are bare" fic...