I'll somewhat disagree with Abigail-II and Elian.
Yes, if you want to interface Perl5 to a C library, then doing so via Inline::C is more likely to make it easy to port to Perl6 than using XS would (the XS porting might be available sooner, but you have much more risk of doing something that won't be handled if you use XS, IMHO).
YHO would be very incorrect in this case. The issues involved are entirely those of the perl API, and have nothing to do with the means to generate the C code to access that API. There are no problems one will encounter with XS that one won't in Inline::C, and vice versa.
The means of generating the C code has no bearing on the potential level of trouble. The issues are entirely ones of the perl API.