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 $77.78   9.747%   $85.3612   $85.36   $85.35   $85.40  
  Purchase #2 $81.58   9.723%   $89.512   $89.51   $89.50   $89.50  
  Purchase #3 $141.57   5.431%   $149.2587   $149.26   $149.25   $149.30  
  Purchase #4 $176.30   3.107%   $181.7776   $181.78   $181.80   $181.80  
  Purchase #5 $52.37   10.113%   $57.6662   $57.67   $57.65   $57.70  
  Purchase #6 $5.96   7.309%   $6.3956   $6.40   $6.40   $6.40  
  Purchase #7 $63.42   11.371%   $70.6315   $70.63   $70.65   $70.60  
  7 purchase totals = $598.98       $640.6028   $640.61   $640.60   $640.70