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 $133.26   4.199%   $138.8556   $138.86   $138.85   $138.90  
  Purchase #2 $75.57   5.701%   $79.8782   $79.88   $79.90   $79.90  
  Purchase #3 $79.89   7.461%   $85.8506   $85.85   $85.85   $85.90  
  Purchase #4 $65.58   8.489%   $71.1471   $71.15   $71.15   $71.10  
  Purchase #5 $152.65   13.111%   $172.6639   $172.66   $172.65   $172.70  
  Purchase #6 $66.67   11.392%   $74.265   $74.27   $74.25   $74.30  
  Purchase #7 $15.60   12.279%   $17.5155   $17.52   $17.50   $17.50  
  7 purchase totals = $589.22       $640.1759   $640.19   $640.15   $640.30