Should one knit into the front or back of a stitch when the previous row contained both knit and purl stitches?

There are at least two ways to interpret this question. Taken literally, you're talking about whether to insert the needle into the stitch knit-wise (from front to back) vs purl-wise (from back to front).

Whether to knit into the front or back of a stitch is not related to what stitches occurred in the row below, but to what stitches you want to create in the current row. If you wish to make a knit stitch, insert the needle from front to back into the stitch to be worked. If you wish to make a purl stitch, insert the needle from the back to the front. You are allowed to put purl stitches on top of knit stitches and vice versa. It all depends on what pattern or effect you are looking for. For example, many garments begin with a couple of inches of ribbing, such as 1 x 1 ribbing which is done by alternately knitting a stitch, then purling the next stitch. On following rows you simple knit those stitches that were previously knit and purl those stitches that were previously purled. But at the end of the ribbed cuff, hem, or neck, you will need to switch from ribbing to stockinette or some other pattern stitch. Switching to stockinette would mean knitting all the stitches all the way across, even if some of them had been purled in a previous row. ----- The other way to interpret the question is whether you knit into the leg of the stitch that sits in front of or behind the needle, rather than whether you insert the needle from the front of the work or the back of the work.

When you wrap the yarn around the needle, you might wrap it counter-clockwise if you are a western knitter (Western Europe and the U.S.), or clock-wise if you are an eastern knitter (Asia). Some knitters wrap CCW on the knits and CW on the purls (called "combination" knitting) because this is the most ergonomic technique.

How you wrap the stitch determines how the stitch is placed on the needle. For most American/European knitters the stitch is placed so that the "leading leg" or the branch of the stitch closer to the tip of the needle, is in front of the needle, and the trailing leg is behind the needle. If you knit into the front (leading) leg, you will get a flat stitch. But if you knit into the back (trailing) leg, you will get a twisted stitch.

It can be tempting to simply say "always knit into the front of the stitch," but as you can see that would only be true for half of the world's knitters. It is more accurate to say "knit into the leading leg for a flat stitch or the trailing leg for a twisted stitch." This is always true, regardless of whether the stitch being knit is a purl stitch or a knit stitch and regardless of the knitter's style of knitting.

Each row is treated as a whole new beginning. The stitch would be made the same way each time no matter where you make it, according to what the pattern says. Some advanced knitters do prefer to start and end every row with the same stitch, regardless of what the pattern says because it makes a kind of seam which makes stitching a garment together.