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 $73.89   6.222%   $78.4874   $78.49   $78.50   $78.50  
  Purchase #2 $77.64   7.431%   $83.4094   $83.41   $83.40   $83.40  
  Purchase #3 $29.70   6.133%   $31.5215   $31.52   $31.50   $31.50  
  Purchase #4 $133.62   9.134%   $145.8249   $145.82   $145.80   $145.80  
  Purchase #5 $128.95   3.610%   $133.6051   $133.61   $133.60   $133.60  
  Purchase #6 $7.57   5.670%   $7.9992   $8.00   $8.00   $8.00  
  Purchase #7 $173.39   9.575%   $189.9921   $189.99   $190.00   $190.00  
  7 purchase totals = $624.76       $670.8396   $670.84   $670.80   $670.80