Quick Memory Question

I´m fixing a family members computer and when I got here noticed that the two old memory sticks that were in slot 1 and two have now been replaced with a single 1GB stick of memory. To make a long story short the pc came with one stick of memory from the factory originally seated in slot 1, I purchased a second stick for them later on which I placed in slot 2. The total ended up being somewhere in the 700mb of memory range. But getting here I saw those two sticks were taken out and the single 1GB stick was now in slot 2.

Is there a reason to leave a single memory stick in slot 2?

The supposed tech who checked this computer also left the optical drive disconnected among other things. I just happened to place the new 1GB stick in slot 2* last night and tested it with Memtest86+ and woke up to see about 12 passes without any errors at all.

Safe to say it's okay to leave it in slot 1? I can't understand what if any performance issue would rely on having a single stick stay in slot 2. I'm assuming slot 1 is always the primary and fastest slot to get information to-from.

Thanks