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 $242.35   12.864%   $273.5259   $273.53   $273.55   $273.50  
  Purchase #2 $159.79   8.583%   $173.5048   $173.50   $173.50   $173.50  
  Purchase #3 $242.52   5.698%   $256.3388   $256.34   $256.35   $256.30  
  Purchase #4 $203.53   8.725%   $221.288   $221.29   $221.30   $221.30  
  Purchase #5 $62.29   8.874%   $67.8176   $67.82   $67.80   $67.80  
  Purchase #6 $111.53   10.167%   $122.8693   $122.87   $122.85   $122.90  
  Purchase #7 $152.69   11.234%   $169.8432   $169.84   $169.85   $169.80  
  7 purchase totals = $1,174.70       $1,285.1876   $1,285.19   $1,285.20   $1,285.10