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 $84.82   10.136%   $93.4174   $93.42   $93.40   $93.40  
  Purchase #2 $101.25   11.258%   $112.6487   $112.65   $112.65   $112.60  
  Purchase #3 $232.91   8.192%   $251.99   $251.99   $252.00   $252.00  
  Purchase #4 $11.98   7.195%   $12.842   $12.84   $12.85   $12.80  
  Purchase #5 $89.68   7.258%   $96.189   $96.19   $96.20   $96.20  
  Purchase #6 $69.28   9.821%   $76.084   $76.08   $76.10   $76.10  
  Purchase #7 $31.47   13.694%   $35.7795   $35.78   $35.80   $35.80  
  7 purchase totals = $621.39       $678.9506   $678.95   $679.00   $678.90