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 $70.47   13.752%   $80.161   $80.16   $80.15   $80.20  
  Purchase #2 $52.28   3.360%   $54.0366   $54.04   $54.05   $54.00  
  Purchase #3 $48.77   9.540%   $53.4227   $53.42   $53.40   $53.40  
  Purchase #4 $160.85   7.660%   $173.1711   $173.17   $173.15   $173.20  
  Purchase #5 $129.85   9.662%   $142.3961   $142.40   $142.40   $142.40  
  Purchase #6 $22.70   5.575%   $23.9655   $23.97   $23.95   $24.00  
  Purchase #7 $111.70   3.121%   $115.1862   $115.19   $115.20   $115.20  
  7 purchase totals = $596.62       $642.3392   $642.35   $642.30   $642.40