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 $8.70   4.837%   $9.1208   $9.12   $9.10   $9.10  
  Purchase #2 $21.76   5.357%   $22.9257   $22.93   $22.95   $22.90  
  Purchase #3 $138.82   12.170%   $155.7144   $155.71   $155.70   $155.70  
  Purchase #4 $83.93   5.355%   $88.4245   $88.42   $88.40   $88.40  
  Purchase #5 $145.89   13.505%   $165.5924   $165.59   $165.60   $165.60  
  Purchase #6 $101.32   7.439%   $108.8572   $108.86   $108.85   $108.90  
  Purchase #7 $171.78   10.862%   $190.4387   $190.44   $190.45   $190.40  
  7 purchase totals = $672.20       $741.0737   $741.07   $741.05   $741.00