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 $80.87   11.506%   $90.1749   $90.17   $90.15   $90.20  
  Purchase #2 $164.86   7.621%   $177.424   $177.42   $177.40   $177.40  
  Purchase #3 $135.95   6.717%   $145.0818   $145.08   $145.10   $145.10  
  Purchase #4 $177.72   12.331%   $199.6347   $199.63   $199.65   $199.60  
  Purchase #5 $42.25   10.130%   $46.5299   $46.53   $46.55   $46.50  
  Purchase #6 $210.37   11.170%   $233.8683   $233.87   $233.85   $233.90  
  Purchase #7 $91.68   7.815%   $98.8448   $98.84   $98.85   $98.80  
  7 purchase totals = $903.70       $991.5584   $991.54   $991.55   $991.50