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 $120.30   9.838%   $132.1351   $132.14   $132.15   $132.10  
  Purchase #2 $180.84   4.548%   $189.0646   $189.06   $189.05   $189.10  
  Purchase #3 $123.74   7.773%   $133.3583   $133.36   $133.35   $133.40  
  Purchase #4 $122.58   9.156%   $133.8034   $133.80   $133.80   $133.80  
  Purchase #5 $224.92   6.294%   $239.0765   $239.08   $239.10   $239.10  
  Purchase #6 $38.88   7.828%   $41.9235   $41.92   $41.90   $41.90  
  Purchase #7 $45.96   7.350%   $49.3381   $49.34   $49.35   $49.30  
  7 purchase totals = $857.22       $918.6995   $918.70   $918.70   $918.70