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 $9.40   8.315%   $10.1816   $10.18   $10.20   $10.20  
  Purchase #2 $79.73   13.390%   $90.4058   $90.41   $90.40   $90.40  
  Purchase #3 $75.30   3.180%   $77.6945   $77.69   $77.70   $77.70  
  Purchase #4 $16.66   10.780%   $18.4559   $18.46   $18.45   $18.50  
  Purchase #5 $71.59   3.781%   $74.2968   $74.30   $74.30   $74.30  
  Purchase #6 $128.48   8.837%   $139.8338   $139.83   $139.85   $139.80  
  Purchase #7 $173.52   9.366%   $189.7719   $189.77   $189.75   $189.80  
  7 purchase totals = $554.68       $600.6403   $600.64   $600.65   $600.70