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 $35.42   9.813%   $38.8958   $38.90   $38.90   $38.90  
  Purchase #2 $28.82   5.738%   $30.4737   $30.47   $30.45   $30.50  
  Purchase #3 $191.74   9.288%   $209.5488   $209.55   $209.55   $209.50  
  Purchase #4 $185.30   6.456%   $197.263   $197.26   $197.25   $197.30  
  Purchase #5 $142.49   10.166%   $156.9755   $156.98   $157.00   $157.00  
  Purchase #6 $51.44   3.386%   $53.1818   $53.18   $53.20   $53.20  
  Purchase #7 $42.89   3.630%   $44.4469   $44.45   $44.45   $44.40  
  7 purchase totals = $678.10       $730.7855   $730.79   $730.80   $730.80