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 $78.40   12.454%   $88.1639   $88.16   $88.15   $88.20  
  Purchase #2 $245.94   11.278%   $273.6771   $273.68   $273.70   $273.70  
  Purchase #3 $79.55   3.342%   $82.2086   $82.21   $82.20   $82.20  
  Purchase #4 $149.76   12.116%   $167.9049   $167.90   $167.90   $167.90  
  Purchase #5 $227.51   5.281%   $239.5248   $239.52   $239.50   $239.50  
  Purchase #6 $127.78   11.645%   $142.66   $142.66   $142.65   $142.70  
  Purchase #7 $53.80   7.866%   $58.0319   $58.03   $58.05   $58.00  
  7 purchase totals = $962.74       $1,052.1712   $1,052.16   $1,052.15   $1,052.20