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 $235.52   12.226%   $264.3147   $264.31   $264.30   $264.30  
  Purchase #2 $216.96   11.524%   $241.9625   $241.96   $241.95   $242.00  
  Purchase #3 $190.53   9.314%   $208.276   $208.28   $208.30   $208.30  
  Purchase #4 $148.46   13.330%   $168.2497   $168.25   $168.25   $168.20  
  Purchase #5 $227.54   9.816%   $249.8753   $249.88   $249.90   $249.90  
  Purchase #6 $124.26   4.663%   $130.0542   $130.05   $130.05   $130.10  
  Purchase #7 $5.47   4.387%   $5.71   $5.71   $5.70   $5.70  
  7 purchase totals = $1,148.74       $1,268.4424   $1,268.44   $1,268.45   $1,268.50