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 $20.53   6.565%   $21.8778   $21.88   $21.90   $21.90  
  Purchase #2 $136.64   13.720%   $155.387   $155.39   $155.40   $155.40  
  Purchase #3 $52.75   13.758%   $60.0073   $60.01   $60.00   $60.00  
  Purchase #4 $123.39   13.788%   $140.403   $140.40   $140.40   $140.40  
  Purchase #5 $200.57   9.664%   $219.9531   $219.95   $219.95   $220.00  
  Purchase #6 $29.88   13.666%   $33.9634   $33.96   $33.95   $34.00  
  Purchase #7 $81.34   9.514%   $89.0787   $89.08   $89.10   $89.10  
  7 purchase totals = $645.10       $720.6703   $720.67   $720.70   $720.80