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 $121.86   5.788%   $128.9133   $128.91   $128.90   $128.90  
  Purchase #2 $31.89   5.527%   $33.6526   $33.65   $33.65   $33.70  
  Purchase #3 $151.66   11.508%   $169.113   $169.11   $169.10   $169.10  
  Purchase #4 $149.80   7.245%   $160.653   $160.65   $160.65   $160.70  
  Purchase #5 $173.73   6.775%   $185.5002   $185.50   $185.50   $185.50  
  Purchase #6 $177.74   13.459%   $201.662   $201.66   $201.65   $201.70  
  Purchase #7 $128.59   10.251%   $141.7718   $141.77   $141.75   $141.80  
  7 purchase totals = $935.27       $1,021.2659   $1,021.25   $1,021.20   $1,021.40