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 $240.42   12.168%   $269.6743   $269.67   $269.65   $269.70  
  Purchase #2 $28.36   5.153%   $29.8214   $29.82   $29.80   $29.80  
  Purchase #3 $145.53   5.613%   $153.6986   $153.70   $153.70   $153.70  
  Purchase #4 $137.42   7.119%   $147.2029   $147.20   $147.20   $147.20  
  Purchase #5 $134.97   11.528%   $150.5293   $150.53   $150.55   $150.50  
  Purchase #6 $54.91   5.666%   $58.0212   $58.02   $58.00   $58.00  
  Purchase #7 $119.44   10.727%   $132.2523   $132.25   $132.25   $132.30  
  7 purchase totals = $861.05       $941.2000   $941.19   $941.15   $941.20