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 $228.70   13.233%   $258.9639   $258.96   $258.95   $259.00  
  Purchase #2 $249.98   10.331%   $275.8054   $275.81   $275.80   $275.80  
  Purchase #3 $159.30   11.431%   $177.5096   $177.51   $177.50   $177.50  
  Purchase #4 $230.31   11.710%   $257.2793   $257.28   $257.30   $257.30  
  Purchase #5 $250.70   3.393%   $259.2063   $259.21   $259.20   $259.20  
  Purchase #6 $11.75   12.524%   $13.2216   $13.22   $13.20   $13.20  
  Purchase #7 $242.25   11.174%   $269.319   $269.32   $269.30   $269.30  
  7 purchase totals = $1,372.99       $1,511.3051   $1,511.31   $1,511.25   $1,511.30