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 $240.96   6.203%   $255.9067   $255.91   $255.90   $255.90  
  Purchase #2 $65.27   7.456%   $70.1365   $70.14   $70.15   $70.10  
  Purchase #3 $120.64   8.236%   $130.5759   $130.58   $130.60   $130.60  
  Purchase #4 $119.93   4.708%   $125.5763   $125.58   $125.60   $125.60  
  Purchase #5 $149.36   10.840%   $165.5506   $165.55   $165.55   $165.60  
  Purchase #6 $23.62   8.522%   $25.6329   $25.63   $25.65   $25.60  
  Purchase #7 $145.52   4.825%   $152.5413   $152.54   $152.55   $152.50  
  7 purchase totals = $865.30       $925.9202   $925.93   $926.00   $925.90