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 $86.54   12.458%   $97.3212   $97.32   $97.30   $97.30  
  Purchase #2 $55.64   9.499%   $60.9252   $60.93   $60.95   $60.90  
  Purchase #3 $17.53   3.173%   $18.0862   $18.09   $18.10   $18.10  
  Purchase #4 $155.70   4.784%   $163.1487   $163.15   $163.15   $163.10  
  Purchase #5 $216.38   9.138%   $236.1528   $236.15   $236.15   $236.20  
  Purchase #6 $240.26   7.492%   $258.2603   $258.26   $258.25   $258.30  
  Purchase #7 $149.84   13.120%   $169.499   $169.50   $169.50   $169.50  
  7 purchase totals = $921.89       $1,003.3934   $1,003.40   $1,003.40   $1,003.40