I've implemented some control over minor-ticks in a branch of sage available here. Hopefully it (or some improved version of it) will get incorporated into the sage codebase at some point.

Here's an example which should achieve what was sought after by the questioner (except that minor ticks are smaller than major ticks, which may not have been desired) :

```
var('x')
p=plot(5*sin(x/10)*sin(x),(x,0,20*pi))
p.show(ticks=[[0,5*pi,10*pi,15*pi,20*pi],5], minor_ticks=pi,
tick_formatter="latex")
```

If the `minor_ticks`

option is **not** specified, then minor-tick behavior will follow previous Sage defaults.
If the option **is** specified, then the following examples describe the behavior:

```
minor_ticks=None, or =[None,None] -- no minor ticks will appear on either x or y axes
minor_ticks=3 -- minor ticks every 3 units on x axis ; Sage defaults on y axis
minor_ticks=[3,None] -- minor ticks every 3 units on x axis ; none on y axis
minor_ticks=[3,[2,7]] -- minor ticks every 3 units on x axis ; at 2 and 7 on y axis
```