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 $241.74   13.674%   $274.7955   $274.80   $274.80   $274.80  
  Purchase #2 $70.82   5.495%   $74.7116   $74.71   $74.70   $74.70  
  Purchase #3 $119.53   10.589%   $132.187   $132.19   $132.20   $132.20  
  Purchase #4 $174.85   13.142%   $197.8288   $197.83   $197.85   $197.80  
  Purchase #5 $80.32   8.690%   $87.2998   $87.30   $87.30   $87.30  
  Purchase #6 $132.73   5.644%   $140.2213   $140.22   $140.20   $140.20  
  Purchase #7 $80.56   13.375%   $91.3349   $91.33   $91.35   $91.30  
  7 purchase totals = $900.55       $998.3789   $998.38   $998.40   $998.30