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 $123.77   11.221%   $137.6582   $137.66   $137.65   $137.70  
  Purchase #2 $151.92   5.675%   $160.5415   $160.54   $160.55   $160.50  
  Purchase #3 $79.65   7.659%   $85.7504   $85.75   $85.75   $85.80  
  Purchase #4 $239.86   6.616%   $255.7291   $255.73   $255.75   $255.70  
  Purchase #5 $157.54   6.282%   $167.4367   $167.44   $167.45   $167.40  
  Purchase #6 $97.81   8.483%   $106.1072   $106.11   $106.10   $106.10  
  Purchase #7 $25.80   9.526%   $28.2577   $28.26   $28.25   $28.30  
  7 purchase totals = $876.35       $941.4808   $941.49   $941.50   $941.50