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 $170.40   10.258%   $187.8796   $187.88   $187.90   $187.90  
  Purchase #2 $153.34   8.412%   $166.239   $166.24   $166.25   $166.20  
  Purchase #3 $75.72   5.662%   $80.0073   $80.01   $80.00   $80.00  
  Purchase #4 $142.85   4.403%   $149.1397   $149.14   $149.15   $149.10  
  Purchase #5 $66.69   4.183%   $69.4796   $69.48   $69.50   $69.50  
  Purchase #6 $64.27   8.686%   $69.8525   $69.85   $69.85   $69.90  
  Purchase #7 $7.98   13.276%   $9.0394   $9.04   $9.05   $9.00  
  7 purchase totals = $681.25       $731.6371   $731.64   $731.70   $731.60