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 $246.83   11.845%   $276.067   $276.07   $276.05   $276.10  
  Purchase #2 $166.74   11.618%   $186.1119   $186.11   $186.10   $186.10  
  Purchase #3 $234.55   9.411%   $256.6235   $256.62   $256.60   $256.60  
  Purchase #4 $126.49   8.274%   $136.9558   $136.96   $136.95   $137.00  
  Purchase #5 $84.26   12.846%   $95.084   $95.08   $95.10   $95.10  
  Purchase #6 $189.84   10.602%   $209.9668   $209.97   $209.95   $210.00  
  Purchase #7 $97.59   9.748%   $107.1031   $107.10   $107.10   $107.10  
  7 purchase totals = $1,146.30       $1,267.9121   $1,267.91   $1,267.85   $1,268.00