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 $121.34   11.569%   $135.3778   $135.38   $135.40   $135.40  
  Purchase #2 $92.48   5.404%   $97.4776   $97.48   $97.50   $97.50  
  Purchase #3 $177.41   6.831%   $189.5289   $189.53   $189.55   $189.50  
  Purchase #4 $36.64   4.775%   $38.3896   $38.39   $38.40   $38.40  
  Purchase #5 $151.89   12.683%   $171.1542   $171.15   $171.15   $171.20  
  Purchase #6 $162.96   6.108%   $172.9136   $172.91   $172.90   $172.90  
  Purchase #7 $110.40   13.345%   $125.1329   $125.13   $125.15   $125.10  
  7 purchase totals = $853.12       $929.9746   $929.97   $930.05   $930.00