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 $34.82   13.535%   $39.5329   $39.53   $39.55   $39.50  
  Purchase #2 $226.78   12.723%   $255.6332   $255.63   $255.65   $255.60  
  Purchase #3 $172.29   6.445%   $183.3941   $183.39   $183.40   $183.40  
  Purchase #4 $115.63   7.287%   $124.056   $124.06   $124.05   $124.10  
  Purchase #5 $203.40   4.526%   $212.6059   $212.61   $212.60   $212.60  
  Purchase #6 $5.33   3.187%   $5.4999   $5.50   $5.50   $5.50  
  Purchase #7 $31.80   5.570%   $33.5713   $33.57   $33.55   $33.60  
  7 purchase totals = $790.05       $854.2933   $854.29   $854.30   $854.30