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 $17.50   3.138%   $18.0492   $18.05   $18.05   $18.00  
  Purchase #2 $45.95   12.760%   $51.8132   $51.81   $51.80   $51.80  
  Purchase #3 $20.97   9.738%   $23.0121   $23.01   $23.00   $23.00  
  Purchase #4 $153.26   10.708%   $169.6711   $169.67   $169.65   $169.70  
  Purchase #5 $19.96   6.242%   $21.2059   $21.21   $21.20   $21.20  
  Purchase #6 $227.93   9.657%   $249.9412   $249.94   $249.95   $249.90  
  Purchase #7 $10.61   5.617%   $11.206   $11.21   $11.20   $11.20  
  7 purchase totals = $496.18       $544.8987   $544.90   $544.85   $544.80