Is wet grass actually slipperier than dry grass?
Yes, grass will be more slippery when wet. I was once asked to ferry a 172 up to a friend following some brake maintenance. As I taxiied out along the wet grass runway, it seemed to be dragging just a little to the left. I looked down to notice that the left wheel was not turning. I considered the implication of this to my flight, knowing that both runways I would use were wet grass that morning. I took off with no fuss, and landed at my destination, stopping just short of the paved apron. The left wheel never turned at all (bolted shut by poor maintenance). My friend asked why I did not park it - I winked, and told him he'd soon figure it out!
Where grass can be a lower friction surface than pavement, it can still be higher drag too, just because it may be long. One condition does not necessarily equate to the other directly.
Understanding the friction characteristics of the "runway" you propose to use is important sometimes, but more so, if there is a crosswind. Casual pilots sometimes open the throttle for takeoff, with little apparent consideration for a crosswind (aileron down). On pavement, this may result in a bit of lurch at lift off, and a skipped mainwheel. On a low friction surface, you may have been blown laterally off the runway early in the takeoff roll, if you don't "fly" into the wind on takeoff.
You;ll read the expression: "Fly it from chock to chock". This very true concept applies very certainly to the effects of the wind while the aircraft is moving on the surface. As soon as you are moving, and until you stop, consider the affect of the wind on the aircraft, and the tools you have to maintain control - directional in particular. If your surface is: Grass, wet grass, snow, ice, or water, you have less to no tool in surface friction to assist you in maintaining directional control - so
fly the plane!
You'll be amazed at the great effect of flight controls in maintaining control - even on the surface!