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 $234.84   13.260%   $265.9798   $265.98   $266.00   $266.00  
  Purchase #2 $124.44   8.542%   $135.0697   $135.07   $135.05   $135.10  
  Purchase #3 $226.62   7.606%   $243.8567   $243.86   $243.85   $243.90  
  Purchase #4 $108.42   11.439%   $120.8222   $120.82   $120.80   $120.80  
  Purchase #5 $168.91   12.287%   $189.664   $189.66   $189.65   $189.70  
  Purchase #6 $203.46   10.273%   $224.3614   $224.36   $224.35   $224.40  
  Purchase #7 $51.98   5.457%   $54.8165   $54.82   $54.80   $54.80  
  7 purchase totals = $1,118.67       $1,234.5703   $1,234.57   $1,234.50   $1,234.70