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 $18.29   3.291%   $18.8919   $18.89   $18.90   $18.90  
  Purchase #2 $8.83   4.570%   $9.2335   $9.23   $9.25   $9.20  
  Purchase #3 $64.46   10.625%   $71.3089   $71.31   $71.30   $71.30  
  Purchase #4 $120.95   13.476%   $137.2492   $137.25   $137.25   $137.20  
  Purchase #5 $30.45   13.199%   $34.4691   $34.47   $34.45   $34.50  
  Purchase #6 $250.59   4.840%   $262.7186   $262.72   $262.70   $262.70  
  Purchase #7 $84.33   3.740%   $87.4839   $87.48   $87.50   $87.50  
  7 purchase totals = $577.90       $621.3551   $621.35   $621.35   $621.30