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 $18.90   10.401%   $20.8658   $20.87   $20.85   $20.90  
  Purchase #2 $66.62   4.172%   $69.3994   $69.40   $69.40   $69.40  
  Purchase #3 $150.49   9.839%   $165.2967   $165.30   $165.30   $165.30  
  Purchase #4 $170.45   6.292%   $181.1747   $181.17   $181.15   $181.20  
  Purchase #5 $156.73   5.319%   $165.0665   $165.07   $165.05   $165.10  
  Purchase #6 $35.75   9.643%   $39.1974   $39.20   $39.20   $39.20  
  Purchase #7 $13.81   7.743%   $14.8793   $14.88   $14.90   $14.90  
  7 purchase totals = $612.75       $655.8798   $655.89   $655.85   $656.00