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 $106.79   12.822%   $120.4826   $120.48   $120.50   $120.50  
  Purchase #2 $117.83   10.207%   $129.8569   $129.86   $129.85   $129.90  
  Purchase #3 $109.64   3.285%   $113.2417   $113.24   $113.25   $113.20  
  Purchase #4 $202.68   12.483%   $227.9805   $227.98   $228.00   $228.00  
  Purchase #5 $131.34   3.446%   $135.866   $135.87   $135.85   $135.90  
  Purchase #6 $41.43   11.355%   $46.1344   $46.13   $46.15   $46.10  
  Purchase #7 $221.76   13.114%   $250.8416   $250.84   $250.85   $250.80  
  7 purchase totals = $931.47       $1,024.4037   $1,024.40   $1,024.45   $1,024.40