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 $31.91   13.204%   $36.1234   $36.12   $36.10   $36.10  
  Purchase #2 $8.34   6.182%   $8.8556   $8.86   $8.85   $8.90  
  Purchase #3 $131.47   13.843%   $149.6694   $149.67   $149.65   $149.70  
  Purchase #4 $121.43   13.760%   $138.1388   $138.14   $138.15   $138.10  
  Purchase #5 $243.70   8.259%   $263.8272   $263.83   $263.85   $263.80  
  Purchase #6 $73.30   13.845%   $83.4484   $83.45   $83.45   $83.40  
  Purchase #7 $192.36   3.685%   $199.4485   $199.45   $199.45   $199.40  
  7 purchase totals = $802.51       $879.5113   $879.52   $879.50   $879.40