yeah - skip it. my daughter despised tummy time with a passion so we never did it.
didn't seem that critical for strength development as she was rolling from back to front and back again at 2-1/2mo, started cruising at 7-1/2mo, started walking alternating with crawling at 9-1/2mo, and quit crawling altogether at 10-1/2mo.
the only time she enjoyed being on her tummy was when she was laying on daddy's chest and then she would go crackers, lol.
for strength development, you can't beat the good ol' jolly jumper. my youngest would spend *hours* every day in that thing just going at it like mad. later on, i found out they're not supposed to be in them for more than 20 minutes but i guess if she was okay since she was able to bounce and jump steadily for two hours or more at a time.