Minimum deck width would be the width of the tub basically (that rectangle frame). My downstairs soaking tub is bascially that width only (tile goes up the wall from there). If space is an issue, this works well, but with that in mind, I'd go with the oval. Ok I'd go with the oval either way :) It's really nice to have a little space around you in the tub. And should you have little ones around ever, it's nice for them as well.

We have had both, (oval and oblong) and I really did like that deep oval tub. It's much more comfy to relax in than the oblong. I'm not a huge person, but big (very pregnant) and small (oh the days of size 2) I still liked the oval tub we had in TX best.

Now for reality sake, put as much deck as you can on the tub. It's nice to have a place to set a cup of tea or glass of wine or heck, some candles :) or a basket of towels. That space is nice to have :) You could consider an undermount to give you more deck space (which we're doing on the round tub now) but the only thing I'd say there is that you'd want to make sure that the top is back far enough in the tub surround to give you a nice head rest. Our tub is a sit up (like a hot tub) tub verses a lay back tub (which for me is so much more comfy because I'm short). So we can have that deck pretty tight on the opening.

Sorry, I guess I should clarify. (This is why I linked the product page, because I knew I would not be able to explain this properly.) The oval tub has a "preexisting" sort-of-deck with a flange on three sides, otherwise I would not be so daft as to try to put a shower over it. I like my kitchen ceiling where it is! Here, this is what it looks like:

However, and this is what I was asking about with the deck question, from the instructions it appears that there must be some kind of deck on the fourth side which does not have a tile flange if one does not use the optional (icky) apron, but it does not say how wide this deck must be. We both would rather not have to clamber over a wide margin to get into the tub - DH has to use this for showering every day.

I'm mostly a lay-backer, not a sit-upper - my shoulders and upper back get very very painful so I have to get them into the scalding water. Unfortunately there isn't enough room for the 5'5" tub that would let me keep both shoulders and knees underwater and actually cover up most of the protruding bits on the front, leaving pretty much just my head and hands (to hold my book, of course) out of the water. ;-) The room is only 60" wide - it was carved out of a bedroom - and even if we had budget to redo all the plumbing there isn't room to turn the tub round sideways. I dislike the look of "stuff" cluttering up tub corners so I'm considering a down-low niche for my shampoo/soap/etc. and a high-up one for DH's shower things, and there will pretty much never be any kind of decorative bits on the corners because they'd just get soaked with the shower spray anyway. Well, okay, maybe a rubber ducky. (But no flamingos!) :-)

Those four were pretty much the only tubs I've found that fit the parameters of size/depth, cost (crucial!), and appearance. I'm a short-but-wide person and DH is medium-height(5'9")-and-wide and we both manage OK in a standard 60x32 oblong tub - 34" or 36" would be a nicer but pretty much all I could find in this price point were funky molded-up things that looked like they belonged on the space station. Yuck-O. The deepest of the four tubs is actually the oblong Bancroft at 15" water depth but I'm not thrilled with how it looks, the other three are 14" water depth. The oval looks like it would have less shoulder room but that's not the case? The one oval tub I've seen available for sitting in was just about big enough for a kindergartener to do laps in, so it's pretty useless for comparison!

johnmari,
Is there a reason you haven't considered the hourglass.........looks, comfort ? I ask because I liked this model because of the amount of water it holds (64 gal)and its common size. This is also a deep tub.
salbwil

Primarily its looks, salbwil - I've seen it in person and did not care for it.

For the detail you're thinking (the front end supported on a knee wall basically) if you use 2x4's, per kohler's rough in instructions I'd say your deck would end up extending around 4" from the front face of the tub (including tile, thinset, 1/2" cement board and 2x4 framing).