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 $225.26   10.672%   $249.2997   $249.30   $249.30   $249.30  
  Purchase #2 $108.96   8.227%   $117.9241   $117.92   $117.90   $117.90  
  Purchase #3 $17.44   12.742%   $19.6622   $19.66   $19.65   $19.70  
  Purchase #4 $216.36   4.258%   $225.5726   $225.57   $225.55   $225.60  
  Purchase #5 $197.80   6.788%   $211.2267   $211.23   $211.25   $211.20  
  Purchase #6 $204.44   4.384%   $213.4026   $213.40   $213.40   $213.40  
  Purchase #7 $246.66   8.782%   $268.3217   $268.32   $268.30   $268.30  
  7 purchase totals = $1,216.92       $1,305.4096   $1,305.40   $1,305.35   $1,305.40