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 $28.96   3.561%   $29.9913   $29.99   $30.00   $30.00  
  Purchase #2 $83.33   12.345%   $93.6171   $93.62   $93.60   $93.60  
  Purchase #3 $91.83   5.678%   $97.0441   $97.04   $97.05   $97.00  
  Purchase #4 $104.91   4.557%   $109.6907   $109.69   $109.70   $109.70  
  Purchase #5 $63.98   12.675%   $72.0895   $72.09   $72.10   $72.10  
  Purchase #6 $92.70   10.398%   $102.3389   $102.34   $102.35   $102.30  
  Purchase #7 $165.75   9.405%   $181.3388   $181.34   $181.35   $181.30  
  7 purchase totals = $631.46       $686.1104   $686.11   $686.15   $686.00