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 $239.59   10.520%   $264.7949   $264.79   $264.80   $264.80  
  Purchase #2 $209.49   4.185%   $218.2572   $218.26   $218.25   $218.30  
  Purchase #3 $62.77   10.743%   $69.5134   $69.51   $69.50   $69.50  
  Purchase #4 $192.63   6.610%   $205.3628   $205.36   $205.35   $205.40  
  Purchase #5 $25.54   4.490%   $26.6867   $26.69   $26.70   $26.70  
  Purchase #6 $118.25   10.452%   $130.6095   $130.61   $130.60   $130.60  
  Purchase #7 $58.29   11.518%   $65.0038   $65.00   $65.00   $65.00  
  7 purchase totals = $906.56       $980.2283   $980.22   $980.20   $980.30