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 $244.92   4.591%   $256.1643   $256.16   $256.15   $256.20  
  Purchase #2 $131.40   11.720%   $146.8001   $146.80   $146.80   $146.80  
  Purchase #3 $174.72   5.204%   $183.8124   $183.81   $183.80   $183.80  
  Purchase #4 $221.73   3.655%   $229.8342   $229.83   $229.85   $229.80  
  Purchase #5 $75.34   5.173%   $79.2373   $79.24   $79.25   $79.20  
  Purchase #6 $37.25   6.817%   $39.7893   $39.79   $39.80   $39.80  
  Purchase #7 $80.79   9.712%   $88.6363   $88.64   $88.65   $88.60  
  7 purchase totals = $966.15       $1,024.2739   $1,024.27   $1,024.30   $1,024.20