What you could do is fill in your log book in hours and minutes untill you've got your ATPL and yes, just round up or down to the nearest 5 minutes. Personally, I would be conservative here.
Once you've got your ATP and you don't feel like writing that much, then by all means write in decimals, once again rounding up for block time (personal log book) and rounding down for flight time (company A/C log book)
These are just my personal thoughts. Everybody has his or her own style and that's what flying is about.