Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $109.39   13.683%   $124.3578   $124.36   $124.35   $124.40  
  Purchase #2 $203.37   12.242%   $228.2666   $228.27   $228.25   $228.30  
  Purchase #3 $12.25   8.306%   $13.2675   $13.27   $13.25   $13.30  
  Purchase #4 $184.26   7.595%   $198.2545   $198.25   $198.25   $198.30  
  Purchase #5 $105.36   11.395%   $117.3658   $117.37   $117.35   $117.40  
  Purchase #6 $100.76   3.528%   $104.3148   $104.31   $104.30   $104.30  
  Purchase #7 $237.80   12.642%   $267.8627   $267.86   $267.85   $267.90  
  7 purchase totals = $953.19       $1,053.6897   $1,053.69   $1,053.60   $1,053.90