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 $29.50   4.728%   $30.8948   $30.89   $30.90   $30.90  
  Purchase #2 $27.71   6.735%   $29.5763   $29.58   $29.60   $29.60  
  Purchase #3 $32.34   13.264%   $36.6296   $36.63   $36.65   $36.60  
  Purchase #4 $191.27   10.508%   $211.3687   $211.37   $211.35   $211.40  
  Purchase #5 $33.83   10.341%   $37.3284   $37.33   $37.35   $37.30  
  Purchase #6 $59.79   5.435%   $63.0396   $63.04   $63.05   $63.00  
  Purchase #7 $11.43   3.421%   $11.821   $11.82   $11.80   $11.80  
  7 purchase totals = $385.87       $420.6584   $420.66   $420.70   $420.60