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 $25.80   12.704%   $29.0776   $29.08   $29.10   $29.10  
  Purchase #2 $97.73   11.655%   $109.1204   $109.12   $109.10   $109.10  
  Purchase #3 $221.47   6.301%   $235.4248   $235.42   $235.40   $235.40  
  Purchase #4 $133.42   13.842%   $151.888   $151.89   $151.90   $151.90  
  Purchase #5 $31.33   10.535%   $34.6306   $34.63   $34.65   $34.60  
  Purchase #6 $112.92   13.783%   $128.4838   $128.48   $128.50   $128.50  
  Purchase #7 $108.78   10.716%   $120.4369   $120.44   $120.45   $120.40  
  7 purchase totals = $731.45       $809.0621   $809.06   $809.10   $809.00