As with any investment you should only put in what you can afford to lose. I do believe though that now is as good of a time as any to acquire more steem. Steem dollars are a different matter. The way the steem dollar is pegged to be roughly equal to the U.S. dollar won't be affected by hardfork 20 so it doesn't really make much difference in the long run if you buy steem dollars now or later. (Update: There is a case that could be made that hardfork 20 could have an affect on SBD price. See @newageinv comment below).
Hardfork 20 will be introducing new features to steemit. One of those new features should make new account creation easier without the typical long delays. This could mean more new users competing for the same steem and increasing the demand.
Even with the possible increase in demand from new users I think what will really drive steem demand after hardfork 20 is the introduction of resource credits and discount account creation. More veteran users of steemit will have a brand new part of the steem environment to play with.
Photo Source: steemit.com / @firepower