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 $180.48   6.245%   $191.751   $191.75   $191.75   $191.80  
  Purchase #2 $52.66   6.710%   $56.1935   $56.19   $56.20   $56.20  
  Purchase #3 $229.95   7.280%   $246.6904   $246.69   $246.70   $246.70  
  Purchase #4 $167.33   13.841%   $190.4901   $190.49   $190.50   $190.50  
  Purchase #5 $123.68   9.320%   $135.207   $135.21   $135.20   $135.20  
  Purchase #6 $224.43   13.835%   $255.4799   $255.48   $255.50   $255.50  
  Purchase #7 $162.28   3.158%   $167.4048   $167.40   $167.40   $167.40  
  7 purchase totals = $1,140.81       $1,243.2167   $1,243.21   $1,243.25   $1,243.30