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 $132.97   3.793%   $138.0136   $138.01   $138.00   $138.00  
  Purchase #2 $152.70   10.147%   $168.1945   $168.19   $168.20   $168.20  
  Purchase #3 $137.87   10.622%   $152.5146   $152.51   $152.50   $152.50  
  Purchase #4 $142.88   5.264%   $150.4012   $150.40   $150.40   $150.40  
  Purchase #5 $211.88   8.618%   $230.1398   $230.14   $230.15   $230.10  
  Purchase #6 $25.36   11.606%   $28.3033   $28.30   $28.30   $28.30  
  Purchase #7 $31.42   6.377%   $33.4237   $33.42   $33.40   $33.40  
  7 purchase totals = $835.08       $900.9907   $900.97   $900.95   $900.90