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 $19.71   6.501%   $20.9913   $20.99   $21.00   $21.00  
  Purchase #2 $43.50   9.383%   $47.5816   $47.58   $47.60   $47.60  
  Purchase #3 $95.32   11.711%   $106.4829   $106.48   $106.50   $106.50  
  Purchase #4 $192.70   6.816%   $205.8344   $205.83   $205.85   $205.80  
  Purchase #5 $112.37   4.835%   $117.8031   $117.80   $117.80   $117.80  
  Purchase #6 $96.48   13.740%   $109.7364   $109.74   $109.75   $109.70  
  Purchase #7 $22.81   7.296%   $24.4742   $24.47   $24.45   $24.50  
  7 purchase totals = $582.89       $632.9039   $632.89   $632.95   $632.90