While maybe intuitively useful, a slanted bar can be dangerous because if you need it, you may not have a lot of hand strength, and being likely wet, could just slide down it to the end and lose your grip. I put two bars in my tub/shower area...an L-shaped one on one wall, and a vertical one by the shorter, shower head end. I find it handy to steady myself stepping into and out of the tub. The other one on the long wall helps getting up from sitting in the tub, and with the vertical section, can be used to steady yourself while showering.
If the walls will be tiled, I've used anchors from
www.wingits.com that are ADA certified. You need a diamond core bit to drill (one) hole for each end, but I found that the connection was very secure. Much cheaper to have good blocking to screw into, though.
Height, there's standards that has a range in height. Don't remember the range, but think the upper limit was 36" from the tub floor. What I'd do is sit down on the floor and have someone measure the height of your arm when it reaches for a bar to your side. Then, stand up and see how low you can go and still reach feel comfortable. Then decide what height works for both, if possible.
You also want the bar diameter to be fairly large, and keep track of how far it sticks out from the wall - you need enough room to get your hand easily around it and arthritic hands may not bend too well anymore.