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 $7.65   7.728%   $8.2412   $8.24   $8.25   $8.20  
  Purchase #2 $9.85   10.830%   $10.9168   $10.92   $10.90   $10.90  
  Purchase #3 $23.74   6.460%   $25.2736   $25.27   $25.25   $25.30  
  Purchase #4 $15.39   4.193%   $16.0353   $16.04   $16.05   $16.00  
  Purchase #5 $71.48   12.346%   $80.3049   $80.30   $80.30   $80.30  
  Purchase #6 $222.67   4.218%   $232.0622   $232.06   $232.05   $232.10  
  Purchase #7 $240.62   7.486%   $258.6328   $258.63   $258.65   $258.60  
  7 purchase totals = $591.40       $631.4668   $631.46   $631.45   $631.40