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 $9.63   6.636%   $10.269   $10.27   $10.25   $10.30  
  Purchase #2 $12.37   3.202%   $12.7661   $12.77   $12.75   $12.80  
  Purchase #3 $28.34   12.404%   $31.8553   $31.86   $31.85   $31.90  
  Purchase #4 $21.48   7.494%   $23.0897   $23.09   $23.10   $23.10  
  Purchase #5 $112.70   10.547%   $124.5865   $124.59   $124.60   $124.60  
  Purchase #6 $82.54   7.692%   $88.889   $88.89   $88.90   $88.90  
  Purchase #7 $145.30   9.214%   $158.6879   $158.69   $158.70   $158.70  
  7 purchase totals = $412.36       $450.1435   $450.16   $450.15   $450.30