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 $53.71   12.167%   $60.2449   $60.24   $60.25   $60.20  
  Purchase #2 $108.36   11.135%   $120.4259   $120.43   $120.45   $120.40  
  Purchase #3 $160.41   4.107%   $166.998   $167.00   $167.00   $167.00  
  Purchase #4 $26.54   4.553%   $27.7484   $27.75   $27.75   $27.70  
  Purchase #5 $136.48   3.455%   $141.1954   $141.20   $141.20   $141.20  
  Purchase #6 $35.50   4.789%   $37.2001   $37.20   $37.20   $37.20  
  Purchase #7 $50.64   3.802%   $52.5653   $52.57   $52.55   $52.60  
  7 purchase totals = $571.64       $606.3780   $606.39   $606.40   $606.30