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 $18.40   10.597%   $20.3498   $20.35   $20.35   $20.30  
  Purchase #2 $195.48   7.523%   $210.186   $210.19   $210.20   $210.20  
  Purchase #3 $93.58   6.440%   $99.6066   $99.61   $99.60   $99.60  
  Purchase #4 $25.84   9.706%   $28.348   $28.35   $28.35   $28.30  
  Purchase #5 $109.64   12.185%   $122.9996   $123.00   $123.00   $123.00  
  Purchase #6 $70.43   8.337%   $76.3017   $76.30   $76.30   $76.30  
  Purchase #7 $39.39   9.591%   $43.1679   $43.17   $43.15   $43.20  
  7 purchase totals = $552.76       $600.9596   $600.97   $600.95   $600.90