Currently, a script solution is required to pre-select a sheet different from the first one.
function pageLoadHandler() {
var spreadsheet = $find("<%= RadSpreadsheet1.ClientID %>").get_kendoWidget();
var sheet = spreadsheet.sheetByName("Sheet1");
spreadsheet.activeSheet(sheet);
// Sys.Application.remove_load(pageLoadHandler);
}
Sys.Application.add_load(pageLoadHandler);
This script could be entirely loaded from the server-side following this article on how to properly register a script from the code-behind: