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 $155.79   6.561%   $166.0114   $166.01   $166.00   $166.00  
  Purchase #2 $60.99   13.110%   $68.9858   $68.99   $69.00   $69.00  
  Purchase #3 $141.60   8.790%   $154.0466   $154.05   $154.05   $154.00  
  Purchase #4 $72.67   4.362%   $75.8399   $75.84   $75.85   $75.80  
  Purchase #5 $61.41   9.134%   $67.0192   $67.02   $67.00   $67.00  
  Purchase #6 $110.89   8.856%   $120.7104   $120.71   $120.70   $120.70  
  Purchase #7 $126.73   9.431%   $138.6819   $138.68   $138.70   $138.70  
  7 purchase totals = $730.08       $791.2952   $791.30   $791.30   $791.20