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 $241.94   10.586%   $267.5518   $267.55   $267.55   $267.60  
  Purchase #2 $110.32   5.764%   $116.6788   $116.68   $116.70   $116.70  
  Purchase #3 $130.82   11.594%   $145.9873   $145.99   $146.00   $146.00  
  Purchase #4 $207.84   11.793%   $232.3506   $232.35   $232.35   $232.40  
  Purchase #5 $26.27   10.303%   $28.9766   $28.98   $29.00   $29.00  
  Purchase #6 $108.91   12.739%   $122.784   $122.78   $122.80   $122.80  
  Purchase #7 $12.63   5.540%   $13.3297   $13.33   $13.35   $13.30  
  7 purchase totals = $838.73       $927.6588   $927.66   $927.75   $927.80