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 $84.71   6.845%   $90.5084   $90.51   $90.50   $90.50  
  Purchase #2 $158.82   9.192%   $173.4187   $173.42   $173.40   $173.40  
  Purchase #3 $58.48   13.476%   $66.3608   $66.36   $66.35   $66.40  
  Purchase #4 $137.42   6.384%   $146.1929   $146.19   $146.20   $146.20  
  Purchase #5 $231.85   6.562%   $247.064   $247.06   $247.05   $247.10  
  Purchase #6 $172.61   11.566%   $192.5741   $192.57   $192.55   $192.60  
  Purchase #7 $10.30   12.408%   $11.578   $11.58   $11.60   $11.60  
  7 purchase totals = $854.19       $927.6969   $927.69   $927.65   $927.80