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 $121.50   12.852%   $137.1152   $137.12   $137.10   $137.10  
  Purchase #2 $73.94   9.294%   $80.812   $80.81   $80.80   $80.80  
  Purchase #3 $93.31   7.196%   $100.0246   $100.02   $100.00   $100.00  
  Purchase #4 $213.50   3.513%   $221.0003   $221.00   $221.00   $221.00  
  Purchase #5 $150.44   4.346%   $156.9781   $156.98   $157.00   $157.00  
  Purchase #6 $37.72   5.874%   $39.9357   $39.94   $39.95   $39.90  
  Purchase #7 $71.89   7.839%   $77.5255   $77.53   $77.55   $77.50  
  7 purchase totals = $762.30       $813.3914   $813.40   $813.40   $813.30