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 $17.99   3.744%   $18.6635   $18.66   $18.65   $18.70  
  Purchase #2 $179.89   5.488%   $189.7624   $189.76   $189.75   $189.80  
  Purchase #3 $117.50   4.148%   $122.3739   $122.37   $122.35   $122.40  
  Purchase #4 $150.48   12.331%   $169.0357   $169.04   $169.05   $169.00  
  Purchase #5 $114.82   4.750%   $120.274   $120.27   $120.25   $120.30  
  Purchase #6 $70.92   6.301%   $75.3887   $75.39   $75.40   $75.40  
  Purchase #7 $132.69   9.867%   $145.7825   $145.78   $145.80   $145.80  
  7 purchase totals = $784.29       $841.2807   $841.27   $841.25   $841.40