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 $241.95   12.824%   $272.9777   $272.98   $273.00   $273.00  
  Purchase #2 $22.80   5.102%   $23.9633   $23.96   $23.95   $24.00  
  Purchase #3 $187.99   11.667%   $209.9228   $209.92   $209.90   $209.90  
  Purchase #4 $196.62   7.416%   $211.2013   $211.20   $211.20   $211.20  
  Purchase #5 $173.71   9.132%   $189.5732   $189.57   $189.55   $189.60  
  Purchase #6 $12.58   4.700%   $13.1713   $13.17   $13.15   $13.20  
  Purchase #7 $115.96   4.114%   $120.7306   $120.73   $120.75   $120.70  
  7 purchase totals = $951.61       $1,041.5402   $1,041.53   $1,041.50   $1,041.60