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 $142.30   12.500%   $160.0875   $160.09   $160.10   $160.10  
  Purchase #2 $11.32   5.501%   $11.9427   $11.94   $11.95   $11.90  
  Purchase #3 $203.49   7.431%   $218.6113   $218.61   $218.60   $218.60  
  Purchase #4 $34.48   9.717%   $37.8304   $37.83   $37.85   $37.80  
  Purchase #5 $23.68   8.618%   $25.7207   $25.72   $25.70   $25.70  
  Purchase #6 $167.42   4.192%   $174.4382   $174.44   $174.45   $174.40  
  Purchase #7 $10.65   6.313%   $11.3223   $11.32   $11.30   $11.30  
  7 purchase totals = $593.34       $639.9531   $639.95   $639.95   $639.80