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 $104.36   12.398%   $117.2986   $117.30   $117.30   $117.30  
  Purchase #2 $60.78   7.425%   $65.2929   $65.29   $65.30   $65.30  
  Purchase #3 $128.33   11.192%   $142.6927   $142.69   $142.70   $142.70  
  Purchase #4 $240.51   9.190%   $262.6129   $262.61   $262.60   $262.60  
  Purchase #5 $50.33   7.318%   $54.0131   $54.01   $54.00   $54.00  
  Purchase #6 $26.36   3.187%   $27.2001   $27.20   $27.20   $27.20  
  Purchase #7 $12.46   11.536%   $13.8974   $13.90   $13.90   $13.90  
  7 purchase totals = $623.13       $683.0077   $683.00   $683.00   $683.00