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 $112.64   7.395%   $120.9697   $120.97   $120.95   $121.00  
  Purchase #2 $129.42   6.540%   $137.8841   $137.88   $137.90   $137.90  
  Purchase #3 $142.39   12.820%   $160.6444   $160.64   $160.65   $160.60  
  Purchase #4 $6.73   8.250%   $7.2852   $7.29   $7.30   $7.30  
  Purchase #5 $99.95   3.789%   $103.7371   $103.74   $103.75   $103.70  
  Purchase #6 $179.52   5.480%   $189.3577   $189.36   $189.35   $189.40  
  Purchase #7 $112.97   6.786%   $120.6361   $120.64   $120.65   $120.60  
  7 purchase totals = $783.62       $840.5143   $840.52   $840.55   $840.50