If I get a lower-than-normal temperature, I will test again. If I get a more normal reading, I use the higher one. Different spots in your mouth register different temperatures.. you want to get the thermometer in a "heat pocket". There are two under your tongue, one on each side. When you put the thermometer in your mouth make sure to put it to one side or the other under your tongue and not right in the middle. Always use the same area to take your temp (if you use the left pocket, always use the left pocket).
As for low temps being a problem, they can be, especially if they fall below the coverline or just barely above the coverline before your start your period (not just one day, but several days). It can be a sign that you're not producing enough progesterone, and not enough progesterone causes pregnancy loss since your body isn't sustaining the pregnancy long enough for the placenta to grow and take over.
ETA: Your chart looks fine for now.. as long as your temps don't start dipping down instead of going up, you more than likely don't have a progesterone problem.