In ST 2.1 RC2, the docs say:

majorTickSteps : Number
If minimum and maximum are specified it forces the number of major ticks to the specified value.

But if you search through the code, majorTickSteps/minorTickSize is only declared and never used by any of the API.

You can also verify that it ignores it with this example:

Code:
var chart = new Ext.chart.CartesianChart({
store:Ext.create('Ext.data.JsonStore', {
                fields:['name', 'data1', 'data2', 'data3', 'data4', 'data5'],
                data:[
                    {'name':1, 'data1':10, 'data2':12, 'data3':14, 'data4':8, 'data5':13},
                    {'name':2, 'data1':7, 'data2':8, 'data3':16, 'data4':10, 'data5':3},
                    {'name':3, 'data1':5, 'data2':2, 'data3':14, 'data4':12, 'data5':7},
                    {'name':4, 'data1':2, 'data2':14, 'data3':6, 'data4':1, 'data5':23},
                    {'name':5, 'data1':27, 'data2':38, 'data3':36, 'data4':13, 'data5':33}
                ]
            }),
            axes:[
                {
                    type:'numeric',
                    position:'left',
                    fields:['data1'],
                    minimum:0
                },
                {
                    type:'numeric',
                    position:'bottom',
                    minimum:1,
                    maximum:5,
                    majorTickSteps:3,
                    grid:true,
                    fields:['name']
                }
            ],
            series:[
                {
                    type:'line',
                    style:{
                        stroke:'red',
                        lineWidth:1
                    },
                    xField:'name',
                    yField:'data1'
                }
            ]   
});
Ext.Viewport.setLayout('fit');
Ext.Viewport.add(chart);