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 $30.47   4.440%   $31.8229   $31.82   $31.80   $31.80  
  Purchase #2 $48.83   12.185%   $54.7799   $54.78   $54.80   $54.80  
  Purchase #3 $225.76   4.438%   $235.7792   $235.78   $235.80   $235.80  
  Purchase #4 $15.49   5.759%   $16.3821   $16.38   $16.40   $16.40  
  Purchase #5 $43.89   12.525%   $49.3872   $49.39   $49.40   $49.40  
  Purchase #6 $140.42   11.148%   $156.074   $156.07   $156.05   $156.10  
  Purchase #7 $88.56   10.282%   $97.6657   $97.67   $97.65   $97.70  
  7 purchase totals = $593.42       $641.8910   $641.89   $641.90   $642.00