As long as you use level 50 purples (as opposed to attuned) you can +5 them.
As for the best strategy, since you can only slot 1 of each purple IO per build, you can't use all purples.
So basically it comes down to should you attune, or should you +5, and that answer... depends.
If you are going for set bonuses and plan on exemplaring, then you should attuned so that you get set bonuses at levels below 47. If you are frankenslotting then you're probably better off +5ing to maximize enhancement percentages. Note: procs should always be either the minimum level for the set or attuned, since they provide the same benefit regardless of enhancement level.
I personally attuned anything that has a max level > 30 and isn't a purple/pvp. For any set that has a max level of 30 (I'm looking at you, Siphon Insight) I usually will slot level 30s and +5 to max out enhancement values.
Purple and PvP IO set bonuses remain regardless of the level difference, so it can be worth +5ing some of them (for example, most purple sets only have end redux in 1 piece on a dual aspect, making +5ing that one useful)