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 $20.80   4.843%   $21.8073   $21.81   $21.80   $21.80  
  Purchase #2 $21.47   6.117%   $22.7833   $22.78   $22.80   $22.80  
  Purchase #3 $83.79   3.866%   $87.0293   $87.03   $87.05   $87.00  
  Purchase #4 $121.81   5.252%   $128.2075   $128.21   $128.20   $128.20  
  Purchase #5 $52.84   8.851%   $57.5169   $57.52   $57.50   $57.50  
  Purchase #6 $145.46   8.220%   $157.4168   $157.42   $157.40   $157.40  
  Purchase #7 $74.78   8.227%   $80.9322   $80.93   $80.95   $80.90  
  7 purchase totals = $520.95       $555.6933   $555.70   $555.70   $555.60