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 $245.79   12.243%   $275.8821   $275.88   $275.90   $275.90  
  Purchase #2 $187.93   13.251%   $212.8326   $212.83   $212.85   $212.80  
  Purchase #3 $83.76   9.239%   $91.4986   $91.50   $91.50   $91.50  
  Purchase #4 $171.88   10.340%   $189.6524   $189.65   $189.65   $189.70  
  Purchase #5 $215.37   6.359%   $229.0654   $229.07   $229.05   $229.10  
  Purchase #6 $113.71   4.308%   $118.6086   $118.61   $118.60   $118.60  
  Purchase #7 $231.93   6.207%   $246.3259   $246.33   $246.35   $246.30  
  7 purchase totals = $1,250.37       $1,363.8656   $1,363.87   $1,363.90   $1,363.90