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 $248.83   13.758%   $283.064   $283.06   $283.05   $283.10  
  Purchase #2 $26.51   3.162%   $27.3482   $27.35   $27.35   $27.30  
  Purchase #3 $249.33   13.763%   $283.6453   $283.65   $283.65   $283.60  
  Purchase #4 $36.99   4.837%   $38.7792   $38.78   $38.80   $38.80  
  Purchase #5 $155.51   6.151%   $165.0754   $165.08   $165.10   $165.10  
  Purchase #6 $110.86   5.769%   $117.2555   $117.26   $117.25   $117.30  
  Purchase #7 $90.83   6.529%   $96.7603   $96.76   $96.75   $96.80  
  7 purchase totals = $918.86       $1,011.9279   $1,011.94   $1,011.95   $1,012.00