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 $39.50   7.520%   $42.4704   $42.47   $42.45   $42.50  
  Purchase #2 $152.61   13.203%   $172.7591   $172.76   $172.75   $172.80  
  Purchase #3 $201.74   13.322%   $228.6158   $228.62   $228.60   $228.60  
  Purchase #4 $12.82   9.654%   $14.0576   $14.06   $14.05   $14.10  
  Purchase #5 $147.33   4.146%   $153.4383   $153.44   $153.45   $153.40  
  Purchase #6 $13.86   9.519%   $15.1793   $15.18   $15.20   $15.20  
  Purchase #7 $37.61   12.856%   $42.4451   $42.45   $42.45   $42.40  
  7 purchase totals = $605.47       $668.9656   $668.98   $668.95   $669.00