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 $86.36   4.209%   $89.9949   $89.99   $90.00   $90.00  
  Purchase #2 $80.31   3.576%   $83.1819   $83.18   $83.20   $83.20  
  Purchase #3 $54.50   9.416%   $59.6317   $59.63   $59.65   $59.60  
  Purchase #4 $20.77   8.289%   $22.4916   $22.49   $22.50   $22.50  
  Purchase #5 $15.69   8.235%   $16.9821   $16.98   $17.00   $17.00  
  Purchase #6 $84.39   8.707%   $91.7378   $91.74   $91.75   $91.70  
  Purchase #7 $223.87   4.109%   $233.0688   $233.07   $233.05   $233.10  
  7 purchase totals = $565.89       $597.0888   $597.08   $597.15   $597.10