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 $23.82   13.474%   $27.0295   $27.03   $27.05   $27.00  
  Purchase #2 $144.75   9.743%   $158.853   $158.85   $158.85   $158.90  
  Purchase #3 $96.62   5.206%   $101.65   $101.65   $101.65   $101.70  
  Purchase #4 $175.25   13.810%   $199.452   $199.45   $199.45   $199.50  
  Purchase #5 $38.26   12.245%   $42.9449   $42.94   $42.95   $42.90  
  Purchase #6 $158.77   9.127%   $173.2609   $173.26   $173.25   $173.30  
  Purchase #7 $233.94   5.119%   $245.9154   $245.92   $245.90   $245.90  
  7 purchase totals = $871.41       $949.1057   $949.10   $949.10   $949.20