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 $234.44   6.123%   $248.7948   $248.79   $248.80   $248.80  
  Purchase #2 $163.33   7.463%   $175.5193   $175.52   $175.50   $175.50  
  Purchase #3 $58.73   9.515%   $64.3182   $64.32   $64.30   $64.30  
  Purchase #4 $147.83   8.794%   $160.8302   $160.83   $160.85   $160.80  
  Purchase #5 $12.66   13.709%   $14.3956   $14.40   $14.40   $14.40  
  Purchase #6 $65.38   13.625%   $74.288   $74.29   $74.30   $74.30  
  Purchase #7 $114.64   9.323%   $125.3279   $125.33   $125.35   $125.30  
  7 purchase totals = $797.01       $863.4740   $863.48   $863.50   $863.40