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 $30.68   12.116%   $34.3972   $34.40   $34.40   $34.40  
  Purchase #2 $76.41   9.494%   $83.6644   $83.66   $83.65   $83.70  
  Purchase #3 $145.46   11.361%   $161.9857   $161.99   $162.00   $162.00  
  Purchase #4 $11.91   10.636%   $13.1767   $13.18   $13.20   $13.20  
  Purchase #5 $111.62   12.453%   $125.52   $125.52   $125.50   $125.50  
  Purchase #6 $99.95   6.144%   $106.0909   $106.09   $106.10   $106.10  
  Purchase #7 $223.70   8.561%   $242.851   $242.85   $242.85   $242.90  
  7 purchase totals = $699.73       $767.6859   $767.69   $767.70   $767.80