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 $104.51   13.252%   $118.3597   $118.36   $118.35   $118.40  
  Purchase #2 $25.45   13.380%   $28.8552   $28.86   $28.85   $28.90  
  Purchase #3 $224.36   6.333%   $238.5687   $238.57   $238.55   $238.60  
  Purchase #4 $11.59   6.691%   $12.3655   $12.37   $12.35   $12.40  
  Purchase #5 $26.91   7.649%   $28.9683   $28.97   $28.95   $29.00  
  Purchase #6 $246.53   9.379%   $269.652   $269.65   $269.65   $269.70  
  Purchase #7 $60.70   12.357%   $68.2007   $68.20   $68.20   $68.20  
  7 purchase totals = $700.05       $764.9701   $764.98   $764.90   $765.20