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 $18.93   10.807%   $20.9758   $20.98   $21.00   $21.00  
  Purchase #2 $12.57   7.386%   $13.4984   $13.50   $13.50   $13.50  
  Purchase #3 $30.65   11.653%   $34.2216   $34.22   $34.20   $34.20  
  Purchase #4 $150.33   3.628%   $155.784   $155.78   $155.80   $155.80  
  Purchase #5 $159.83   8.250%   $173.016   $173.02   $173.00   $173.00  
  Purchase #6 $215.40   8.187%   $233.0348   $233.03   $233.05   $233.00  
  Purchase #7 $156.95   5.228%   $165.1553   $165.16   $165.15   $165.20  
  7 purchase totals = $744.66       $795.6859   $795.69   $795.70   $795.70