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 $73.78   7.743%   $79.4928   $79.49   $79.50   $79.50  
  Purchase #2 $27.42   13.711%   $31.1796   $31.18   $31.20   $31.20  
  Purchase #3 $208.93   9.642%   $229.075   $229.08   $229.10   $229.10  
  Purchase #4 $27.38   10.388%   $30.2242   $30.22   $30.20   $30.20  
  Purchase #5 $97.40   8.454%   $105.6342   $105.63   $105.65   $105.60  
  Purchase #6 $13.82   10.654%   $15.2924   $15.29   $15.30   $15.30  
  Purchase #7 $110.30   12.688%   $124.2949   $124.29   $124.30   $124.30  
  7 purchase totals = $559.03       $615.1931   $615.18   $615.25   $615.20