Topbalancing is usually done at the (near) highest allowed voltage of your cells.
For regular LiFePO that's 3.65V, so that is why people use that for topbalancing. 3.6V will work also.
Recommended regular working voltage is lower, while using 3.65V for topbalancing, its generally recommended to use lower voltages for regular use
LiFeYPO has a slightly different voltage curve, if I remember 3.8V. But without the datasheet of your specific cells no-one can recommend you a safe voltage.
Also, topbalancing: Don't make the common mistakes: Adjust PSU before connecting the cells to the proper voltage, use proper leads (many included leads with cheap chargers are crap), double check with a multimeter, never readjust voltage once cells are connected