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 $28.81   10.425%   $31.8134   $31.81   $31.80   $31.80  
  Purchase #2 $16.45   11.826%   $18.3954   $18.40   $18.40   $18.40  
  Purchase #3 $99.62   9.762%   $109.3449   $109.34   $109.35   $109.30  
  Purchase #4 $18.64   6.433%   $19.8391   $19.84   $19.85   $19.80  
  Purchase #5 $12.51   4.169%   $13.0315   $13.03   $13.05   $13.00  
  Purchase #6 $169.70   10.403%   $187.3539   $187.35   $187.35   $187.40  
  Purchase #7 $9.70   12.281%   $10.8913   $10.89   $10.90   $10.90  
  7 purchase totals = $355.43       $390.6695   $390.66   $390.70   $390.60