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 $217.66   8.279%   $235.6801   $235.68   $235.70   $235.70  
  Purchase #2 $120.55   12.810%   $135.9925   $135.99   $136.00   $136.00  
  Purchase #3 $51.69   4.146%   $53.8331   $53.83   $53.85   $53.80  
  Purchase #4 $227.92   13.733%   $259.2203   $259.22   $259.20   $259.20  
  Purchase #5 $180.79   5.768%   $191.218   $191.22   $191.20   $191.20  
  Purchase #6 $61.67   5.855%   $65.2808   $65.28   $65.30   $65.30  
  Purchase #7 $57.30   8.613%   $62.2352   $62.24   $62.25   $62.20  
  7 purchase totals = $917.58       $1,003.4600   $1,003.46   $1,003.50   $1,003.40