You can only upgrade 5* heroes to 6* heroes only.

a general thing for most of us, we only use the rank up tickets or hero type selectors on hard to obtain characters like A-Force girls, Malekith, Angela, Racoon, Black Cat & Jessica Jones, second in line would be those Dimension Rift specific like Destroyer & Loki, third into consideration would be those who had a single mission + rifts to obtain their bios like Black Panther, Thor & Black Bolt.

The rest, we'll farm them daily.

but if you have the urge to rank up a char that you use frequently then just to so as it'll help you progress the game much faster. have a look at the first page, bro David made a list on who's top tier to low tier

if i were u i would go for the rank hard to farm ones. or the ones u like the most.