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 $61.96   6.562%   $66.0258   $66.03   $66.05   $66.00  
  Purchase #2 $10.27   10.686%   $11.3675   $11.37   $11.35   $11.40  
  Purchase #3 $155.51   6.873%   $166.1982   $166.20   $166.20   $166.20  
  Purchase #4 $85.55   9.803%   $93.9365   $93.94   $93.95   $93.90  
  Purchase #5 $91.65   5.234%   $96.447   $96.45   $96.45   $96.40  
  Purchase #6 $70.38   7.532%   $75.681   $75.68   $75.70   $75.70  
  Purchase #7 $83.66   12.164%   $93.8364   $93.84   $93.85   $93.80  
  7 purchase totals = $558.98       $603.4924   $603.51   $603.55   $603.40