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 $129.97   6.602%   $138.5506   $138.55   $138.55   $138.60  
  Purchase #2 $87.84   7.399%   $94.3393   $94.34   $94.35   $94.30  
  Purchase #3 $151.71   11.187%   $168.6818   $168.68   $168.70   $168.70  
  Purchase #4 $199.31   13.607%   $226.4301   $226.43   $226.45   $226.40  
  Purchase #5 $102.39   6.784%   $109.3361   $109.34   $109.35   $109.30  
  Purchase #6 $242.82   3.700%   $251.8043   $251.80   $251.80   $251.80  
  Purchase #7 $43.55   6.373%   $46.3254   $46.33   $46.35   $46.30  
  7 purchase totals = $957.59       $1,035.4676   $1,035.47   $1,035.55   $1,035.40