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 $171.90   4.181%   $179.0871   $179.09   $179.10   $179.10  
  Purchase #2 $201.46   13.761%   $229.1829   $229.18   $229.20   $229.20  
  Purchase #3 $74.30   8.835%   $80.8644   $80.86   $80.85   $80.90  
  Purchase #4 $31.78   11.452%   $35.4194   $35.42   $35.40   $35.40  
  Purchase #5 $227.54   13.848%   $259.0497   $259.05   $259.05   $259.00  
  Purchase #6 $182.33   8.525%   $197.8736   $197.87   $197.85   $197.90  
  Purchase #7 $66.30   4.309%   $69.1569   $69.16   $69.15   $69.20  
  7 purchase totals = $955.61       $1,050.6340   $1,050.63   $1,050.60   $1,050.70