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 $236.56   13.827%   $269.2692   $269.27   $269.25   $269.30  
  Purchase #2 $90.43   13.571%   $102.7023   $102.70   $102.70   $102.70  
  Purchase #3 $135.88   7.442%   $145.9922   $145.99   $146.00   $146.00  
  Purchase #4 $241.51   13.114%   $273.1816   $273.18   $273.20   $273.20  
  Purchase #5 $94.82   12.601%   $106.7683   $106.77   $106.75   $106.80  
  Purchase #6 $222.28   10.171%   $244.8881   $244.89   $244.90   $244.90  
  Purchase #7 $120.47   5.759%   $127.4079   $127.41   $127.40   $127.40  
  7 purchase totals = $1,141.95       $1,270.2096   $1,270.21   $1,270.20   $1,270.30