We use a 10-point scale to rank our perception of a woman's looks, and on occasion will use it to judge her personality as well.
Rarely, the two will be combined on the same 10-point scale (see below).
Normally, the minimum precision used is a half-point (0.5), since the scale is largely used for initial impressions only and these often change anyway based on proximity, make-up, clothing, lighting, inebriation, and a variety of other factors.
Though there is no explicit rule against using higher precision, men who attempt (for example) to rank a girl an 8.4 instead of rounding up to 8.5 are normally laughed at or told to shut the fuck up.
At most, it is a reliable source of advice for women who want to improve their chances with the opposite sex.