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 $25.27   12.765%   $28.4957   $28.50   $28.50   $28.50  
  Purchase #2 $6.54   5.802%   $6.9195   $6.92   $6.90   $6.90  
  Purchase #3 $179.40   8.173%   $194.0624   $194.06   $194.05   $194.10  
  Purchase #4 $171.82   13.368%   $194.7889   $194.79   $194.80   $194.80  
  Purchase #5 $99.25   9.536%   $108.7145   $108.71   $108.70   $108.70  
  Purchase #6 $83.34   5.786%   $88.1621   $88.16   $88.15   $88.20  
  Purchase #7 $165.80   8.258%   $179.4918   $179.49   $179.50   $179.50  
  7 purchase totals = $731.42       $800.6349   $800.63   $800.60   $800.70