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 $20.89   7.688%   $22.496   $22.50   $22.50   $22.50  
  Purchase #2 $8.90   4.385%   $9.2903   $9.29   $9.30   $9.30  
  Purchase #3 $19.39   12.824%   $21.8766   $21.88   $21.90   $21.90  
  Purchase #4 $140.39   5.323%   $147.863   $147.86   $147.85   $147.90  
  Purchase #5 $176.79   10.249%   $194.9092   $194.91   $194.90   $194.90  
  Purchase #6 $6.66   6.778%   $7.1114   $7.11   $7.10   $7.10  
  Purchase #7 $161.64   3.178%   $166.7769   $166.78   $166.80   $166.80  
  7 purchase totals = $534.66       $570.3234   $570.33   $570.35   $570.40