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 $232.87   6.456%   $247.9041   $247.90   $247.90   $247.90  
  Purchase #2 $58.70   13.441%   $66.5899   $66.59   $66.60   $66.60  
  Purchase #3 $121.52   11.722%   $135.7646   $135.76   $135.75   $135.80  
  Purchase #4 $34.52   12.864%   $38.9607   $38.96   $38.95   $39.00  
  Purchase #5 $237.98   6.688%   $253.8961   $253.90   $253.90   $253.90  
  Purchase #6 $143.42   4.234%   $149.4924   $149.49   $149.50   $149.50  
  Purchase #7 $246.30   4.790%   $258.0978   $258.10   $258.10   $258.10  
  7 purchase totals = $1,075.31       $1,150.7056   $1,150.70   $1,150.70   $1,150.80