Its possible and you can and will still have a great trip. The biggest problem is that "temptation" lurks everywhere. Go up to the sail away parties and lots and lots of people have those fru-fru drinks and there are waiters everywhere with trays of them - so easy, so convienient and once you've had one - ohhh so wonderful!!
Then there is the gift shops on the ship. Lots of really nice stuff in them. My families weakness is Shutters. The photog's take really great pictures of the kids and bam, before we know it we've spent $100 and working on the next $100
Dinners are great. All the wonderful food you want. Water, Milk, soda, "normal" coffee

are all available for free. But your assistant server will tempt you with the wine package, a bottled water package or two, a bottle of wine for this dinner, a coctail, a smoothie. Sooner rather than later I break down and buy something (because smoothies are Good!!). And I don't mean to make it sound like they are doing the high pressure sales job on you, just doing their job waiting on you and offering items that you might be interested in.
I won't skip Palo brunch at least once per cruise so we rack up $50 there (and it IS worth it!!)
Oh yea, up on deck 9 by the pools they have mini stores set up with gadgets and toys to tempt the kids and eventually I crumble and buy them something
So yes its possible to just pay for tips while on the cruise, but its hard to stick to just that.