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 $17.91   9.699%   $19.6471   $19.65   $19.65   $19.60  
  Purchase #2 $80.78   8.675%   $87.7877   $87.79   $87.80   $87.80  
  Purchase #3 $151.35   5.610%   $159.8407   $159.84   $159.85   $159.80  
  Purchase #4 $106.69   5.315%   $112.3606   $112.36   $112.35   $112.40  
  Purchase #5 $9.61   8.737%   $10.4496   $10.45   $10.45   $10.40  
  Purchase #6 $117.45   9.683%   $128.8227   $128.82   $128.80   $128.80  
  Purchase #7 $162.25   11.277%   $180.5469   $180.55   $180.55   $180.50  
  7 purchase totals = $646.04       $699.4553   $699.46   $699.45   $699.30